Inside Intel's first product: the 3101 RAM chip held just 64 bits

Intel's first product was not a processor, but a memory chip: the 31011 RAM chip, released in April 1969. This chip held just 64 bits of data (equivalent to 8 letters or 16 digits) and had the steep price tag of $99.50.2 The chip's capacity was way too small to replace core memory, the dominant storage technology at the time, which stored bits in tiny magnetized ferrite cores. However, the 3101 performed at high speed due to its special Schottky transistors, making it useful in minicomputers where CPU registers required fast storage. The overthrow of core memory would require a different technology—MOS DRAM chips—and the 3101 remained in use in the 1980s.3

This article looks inside the 3101 chip and explains how it works. I received two 3101 chips from Evan Wasserman and used a microscope to take photos of the tiny silicon die inside.4 Around the outside of the die, sixteen black bond wires connect pads on the die to the chip's external pins. The die itself consists of silicon circuitry connected by a metal layer on top, which appears golden in the photo. The thick metal lines through the middle of the chip power the chip. The silicon circuitry has a grayish-purple color, but it largely covered by the metal layer. Most of the chip contains a repeated pattern: this is the 16x4 array of storage cells. In the upper left corner of the chip, the digits "3101" in metal identify the chip, but "Intel" is not to be found.

Die photo of the Intel 3101 64-bit RAM chip. Click for a larger image.

Overview of the chip

The 3101 chip is controlled through its 16 external pins. To select one of the chip's 16 words of memory, the address in binary is fed into the chip through the four address pins (A0 to A3). Memory is written by providing the 4-bit value on the data input pins (D1 to D4). Four data output pins (O1 to O4) are used to read memory; these pins are inverted as indicated by the overbar. The chip has two control inputs. The chip select pin (CS) enables or disables the chip. The write enable pin (WE) selects between reading or writing the memory. The chip is powered with 5 volts across the Vcc and ground pins.

The diagram below shows how the key components of the 3101 are arranged on the die. The RAM storage cells are arranged as 16 rows of 4 bits. Each row stores a word, with bits D1 and D2 on the left and D3 and D4 on the right. The address decode logic in the middle selects which row of storage is active, based on the address signals coming from the address drivers at the top. At the bottom, the read/write drivers provide the interface between the storage cells and the data in and out pins.

Block diagram of the 3101 RAM chip.

Transistors

Transistors are the key components in a chip. The 3101 uses NPN bipolar transistors, different from the MOS transistors used in modern memory chips. The diagram below shows one of the transistors in the 3101 as it appears on the die. The slightly different tints in the silicon indicate regions that have been doped to form N and P type silicon with different semiconductor properties. The cross-section diagram illustrates the internal structure of the transistor. On top (black) are the metal contacts for the collector (C), emitter (E), and base (B). Underneath, the silicon has been doped to form the N and P regions that make up the transistor.

A key innovation of the 3101 was using Schottky transistors (details), which made the 3101 almost twice as fast as other memory chips.5 In the cross section, note that the base's metal contact touches both the P and N regions. You might think this shorts the two regions together, but instead a Schottky diode is formed where the metal contacts the N layer.6

The structure of an NPN Schottky transistor inside the Intel 3101 chip.

The 3101 also used many multiple-emitter transistors. While a multiple-emitter transistors may seem strange, they are common in bipolar integrated circuits, especially TTL logic chips. A multiple-emitter transistor simply has several emitter regions embedded in the base region. The die photo below shows one of these transistors with the collector on the left, followed by the base and two emitters.

A multiple-emitter transistor from the Intel 3101 chip.

Driving the data output pins requires larger, high-current transistors. The image below shows one of these transistors. The central rectangle is the base, surrounded by the C-shaped emitter in the middle and the large collector on the outside. Eight of these high-current transistors are also used to drive the internal address select lines.

For the high-current output, the Intel 3101 chip uses larger transistors.

Diodes

While examining the 3101 chip, I was surprised by the large number of diodes on the chip. Eventually I figured out that the chip used DTL (diode-transistor logic) for most of its logic rather than TTL (transistor-transistor logic) that I was expecting. The diagram below shows one of the diodes on the chip. I believe the chip builds diodes using the standard technique of connecting an NPN transistor as a diode.

Presumed structure of a diode inside the 3101 chip. I believe this is a regular diode, not a Schottky diode.

Resistors

The die photo below shows several resistors on the 3101 die. The long, narrow snaking regions of p-type silicon provide resistance. Resistors in integrated circuits are inconveniently large, but are heavily used in the 3101 for pull-up and pull-down resistors. At the right is a square resistor, which has low resistance because it is very wide.7 It is used to route a signal under the metal layer, rather than functioning a resistor per se.

Resistors inside the 3101 chip.

The static RAM cell

Now that I've explained the individual components of the chip, I'll explain how the circuitry is wired together for storage. The diagram below shows the cell for one bit of storage with the circuit diagram overlaid. Each cell consists of two multi-emitter transistors (outlined in red) and two resistors (at the top). The horizontal and vertical wiring connects cells together. This circuit forms a static RAM cell, basically a latch that can be in one of two states, storing one data bit.

The circuitry of one storage cell of the 3101 RAM chip. The two multiple-emitter transistors are outlined in red.

Before explaining how this storage cell works, I'll explain a simpler latch circuit, below. This circuit has two transistors cross-connected so if one transistor is on, it forces the other off. In the diagram, the left transistor is on, which keeps the right transistor off, which keeps the left transistor on. Thus, the circuit will remain in this stable configuration. The opposite state—with the left transistor off and the right transistor on—is also stable. Thus, the latch has two stable configurations, allowing it to hold a 0 or a 1.

A simple latch circuit. The transistor on the left is on, forcing the transistor on the right off, forcing the transistor on the left off...

To make this circuit usable—so the bit can be read or modified—more complex transistors with two emitters are used. One emitter is used to select which cell to read or write, while the other emitter is used for the read or write data. This yields the schematic below, which matches the storage cell die photo diagram above.

The RAM cell used in the Intel 3101 is based on multiple-emitter transistors. The row select lines are raised to read/write the row of cells. Each data line accesses a column of cells.

Multiple storage cells are combined into a grid to form the memory memory. One word of memory consists of cells in the same row that share select lines. All the cells in a column store the same bit position; their data lines are tied together. (The bias line provides a voltage level to all cells in the memory.8)

Note that unlike the simplified cell, the circuit above doesn't have an explicit ground connection; to be powered, it requires a low input on either the select or data/bias lines. There are three cases of interest:

• Unselected: If the negative row select line is low, current flows out through the row select line. The data and bias lines are unaffected by this cell.
• Read: If the negative row select line is higher than the data and bias lines, current will flow out the data line if the left transistor is on, and out the bias line if the right transistor is on. Thus, the state of the cell can be read by examining the current on the data line.
• Write: If the negative row select line is higher and the data and bias lines have significantly different voltages, the transistor on the lower side will switch on, forcing the cell into a particular state. This allows a 0 or 1 to be written to the cell.

Thus, by carefully manipulating the voltages on the select lines, data lines and the bias line, one row of memory can be read or written, while the other cells hold their current value without influencing the data line. The storage cell and the associated read/write circuitry are essentially analog circuits rather than digital since the select, data, and bias voltages must be carefully controlled voltages rather than logic levels.

The address decode logic

The address decode circuitry determines which row of memory cells is selected by the address lines.11 The interesting thing about this circuitry is that you can easily see how it works just by looking at the die photo. The address driver circuitry sends the four address signals along with their complements on eight metal traces through the chip. Each storage row has a four-emitter transistor. In each row you can see four black dots, which are the connections between emitters and address lines. A row will be selected if all the emitter inputs are high.9 A dot on an address line (e.g. A0) will "match" a 1, while a dot on the complemented address line (e.g. A0) will match a 0, so each row matches a unique four-bit address. In the die photo below, you can see the decoding logic counting down in binary for rows 15 down to 11;10 the remainder of the circuit follows the same pattern.

The address decode logic in the Intel 3101 RAM chip. Each row decodes matches four address lines to decode one of the 16 address combinations. You can see the value counting down in binary.

Some systems that used the 3101

The 64-bit storage capacity of the 3101 was too small for a system's main memory, but the chip had a role in many minicomputers. For example, the Burroughs D Machine was a military computer (and the source of the chips I examined). It used core memory for its main storage, but a board full of 3101 chips provided high-speed storage for its microcode. The Xerox Alto used four 3101 chips to provide 16 high-speed registers for the CPU, while the main memory used slower DRAM chips. Interdata used 3101 chips in many of its 16- and 32-bit minicomputers up until the 1980s.12

The 3101 was also used in smaller systems. The Diablo 8233 terminal used them as RAM.13 The Datapoint 2200 was a "programmable terminal" that held its processor stack in fast 3101 chips rather than the slow main memory which was built from Intel 1405 shift registers.

The CPU of the Datapoint 2200 computer was built from a board full of TTL chips. The four white chips in the lower center-right are Intel 3101 RAM chips holding the stack. Photo courtesy of Austin Roche (I think).

How I created the die photos

To get the die photos, I started with two chips that I received thanks to Evan Wasserman and John Culver. The pins on the chips had been crushed in the mail, but this didn't affect the die photos. The chips had two different lot numbers that indicate they were manufactured a few months apart. Strangely, the metal lids on the chips were different sizes and the dies were slightly different. For more information, see the CPU Shack writeup of the 3101.

Two 3101 RAM chips. The chip on the right was manufactured slightly later and has a larger lid over the die.

Popping the metal lid off the chips was easy—just a tap with a hammer and chisel. This revealed the die inside.

With the lid removed, you can see the die of the 3101 RAM chip and the bond wires connected to the die.

Using a metallurgical microscope and Hugin stitching software (details), I stitched together multiple microscope photos to create an image of the die. The metal layer is clearly visible, but it obscures the silicon underneath, making it hard to determine the chip's circuitry. The photo below shows a closeup of the die showing the "3101" part number.

The die photo of the Intel 3101 shows mostly the metal layer.

I applied acid14 to remove the metal layer. This removed most of the metal, revealing the silicon circuitry underneath. Some of the metal is still visible, but thinner, appearing transparent green. Strangely, the number 3101 turned into 101; apparently the first digit wasn't as protected by oxide as the other digits.

Treatment with acid dissolved most of the metal layer of the 3101 chip, revealing the silicon circuits underneath.

Below is the complete die photo of the chip with the metal layer partially stripped off. (Click it for a larger version.) This die photo was most useful for analyzing the chip. Enough of the metal was removed to clearly show the silicon circuits, but the remaining traces of metal showed most of the wiring. The N+ silicon regions appear to have darkened in this etch cycle.

Die photo of the Intel 3101 64-bit RAM chip with metal layer partially stripped off.

I wanted to see how the chip looked with the metal entirely removed so I did a second etch cycle. Unfortunately, this left the die looking like it had been destroyed.

After dissolving most of the oxide layer, the die looks like a mess. (This is a different region from the other photos.)

I performed a third etch cycle. It turns out that the previous etch hadn't destroyed the die, but just left a thin layer of oxide that caused colored interference bands. The final etch removed the remaining oxide, leaving a nice, clean die. Only a ghost of the "101" number is visible. The contacts between the metal layer and the silicon remained after the etch; they may be different type of metal that didn't dissolve.

The metal and oxide have been completely removed from the 3101 die, showing the silicon layer.

Below is the full die photo with all the metal stripped off. (Click it for a full-size image.)

Die photo of the Intel 3101 64-bit RAM chip with metal layer stripped off.

Conclusion

The 3101 RAM chip illustrates the amazing improvements in integrated circuits driven by Moore's Law.15 While the 3101 originally cost $99.50 for 64 bits, you can now buy 16 gigabytes of RAM for that price, two billion times as much storage. If you built a 16 GB memory from two billion 3101 chips, the chips alone would weigh about 3000 tons and use over a billion watts, half of Hoover Dam's power. A modern 16GB DRAM module, in comparison, uses only about 5 watts.

As for Intel, the 3101 RAM was soon followed by many other memory products with rapidly increasing capacity, making Intel primarily a memory company that also produced processors. However, facing strong competition from Japanese memory manufacturers, Intel changed its focus to microprocessors and abandoned the DRAM business in 1985.16 By 1992, the success of the x86 processor line had made Intel the largest chip maker, justifying this decision. Even though Intel is now viewed as a processor company, it was the humble 3101 memory chip that gave Intel its start.

Thanks to Evan Wasserman and John Culver for sending me the chips. John also did a writeup of the 3101 chip, which you can read at CPU Shack.

Notes and references

1. You might wonder why Intel's first chip had the seemingly-arbitrary number 3101. Intel had a highly-structured naming system. A 3xxx part number indicated a bipolar product. A 1 for the second digit indicated RAM, while the last two digits (01) were a sequence number. Fortunately, the marketing department stepped in and gave the 4004 and 8008 processors better names. ↩

2. Memory chips started out very expensive, but prices rapidly dropped. Computer Design Volume 9 page 28, 1970, announced a price drop of the 3101 from $99.50 to $40 in small volumes. Ironically, the Intel 3101 is now a collector's item and on eBay costs much more than the original price—hundreds of dollars for the right package. ↩

3. Several sources say that the 3101 was the first solid state memory, but this isn't accurate. There were many companies making memory chips in the 1960s. For instance, Texas Instruments announced the 16-bit SN5481 bipolar memory chip in 1966 (Electronics, V39 #1, p151) and Transitron had the TMC 3162 and 3164 16-bit RAM (Electrical Design News, Volume 11, p14). In 1968, RCA made 72-bit and 288-bit CMOS memories for the Air Force (document, photo). Lee Boysel built 256-bit dynamic RAMs at Fairchild in 1968 and 1K dynamic RAMs at Four Phase Systems in 1969 (timeline and Boysel presentation). For more information on the history of memory technology, see timeline and History of Semiconductor Engineering, p215. Another source for memory history is To the Digital Age, p193. ↩

4. From my measurements, the 3101 die is about 2.39mm by 3.65mm. Feature size is about 12µm. ↩

5. If you've used TTL chips, you probably used the 74LSxx family. The "S" stands for the Schottky transistors that make these chip fast. These chips were "the single most profitable product line in the history of Texas Instruments" (ref). ↩

6. The Schottky diode in the Schottky transistor is formed between the base and collector. This diode prevents the transistor from becoming saturated, allowing it to switch faster. ↩

7. The resistance of an IC resistor is proportional to the length divided by the width. The sheet resistance of a material is measured in the unusual unit of ohms per square. You might think it should be per square nanometer or square mm or something, but since the resistance depends on the ratio of length to width, the unit cancels out. ↩

8. The bias line is shared by all the cells. For reading, it is set to a low voltage. For writing, it is set to an intermediate voltage: higher than the data 0 voltage, but lower than the data 1 voltage. The bias voltage is controlled by the write enable pin.

   More advanced chips use two data lines instead of a bias line for more sensitivity. A differential amplifier to compare the currents on the two data lines and distinguish the tiny change between a zero bit and a one bit. However, the 3101 uses such high currents internally that this isn't necessary; it can read the data line directly. ↩

9. If my analysis is correct, when a row is selected, the address decode logic raises both the positive row select and negative row select lines by about 0.8 volts (one diode drop). Thus, the cell is still powered by the same voltage differential, but the voltage shift makes the data and bias lines active. ↩

10. Address lines A3 and A2 are reversed in the decoding logic, presumably because it made chip layout simpler. This has no effect on the operation of the chip since it doesn't matter of the physical word order matches the binary order. ↩

11. The 3101 has a chip select pin that makes it easy to combine multiple chips into a larger memory. If this pin is high, the chip will not read or write its contents. One strange thing about the address decoding logic is that each pair of address lines is driven by a NAND gate latch. There's no actual latching happening, so I don't understand why this circuit is used.

    How the 3101 implements this feature is a bit surprising. The chip select signal is fed into the address decoding circuit; if the chip is not selected, both A0 and the complement A0 are forced low. Thus, none of the rows will match in the address decoding logic and the chip doesn't respond. ↩

12. The Interdata 7/32 (the first 32-bit minicomputer) used 3101 chips in its memory controller. (See the maintenance manual page 338.) The Interdata 16/HSALU used 3101 chips for its CPU registers. (See the maintenance manual page 259.) As late as 1982, the Interdata 3210 used 3101 chips to hold cache tags (see manual page 456). On the schematics note that part number 19-075 indicates the 3101. ↩

13. The Diablo 8233 terminal used 3101A (74S289) chips as RAM for its discrete TTL-based processor (which was more of a microcontroller) that controlled the printer. (See maintenance manual page 187.) This systems was unusual since it contained both an 8080 microprocessor and a TTL-based processor. ↩

14. The metal layer of the chip is protected by silicon dioxide passivation layer. The professional way to remove this layer is with dangerous hydrofluoric acid. Instead, I used Armour Etch glass etching cream, which is slightly safer and can be obtained at craft stores. I applied the etching cream to the die and wiped it for four minutes with a Q-tip. (Since the cream is designed for frosting glass, it only etches in spots. It must be moved around to obtain a uniform etch.) Next, I applied a few drops of hydrochloric acid (pool acid from the hardware store) to the die for a few hours. ↩

15. Moore's law not only describes the exponential growth in transistors per chip, but drives this growth. The semiconductor industry sets its roadmap according to Moore's law, making it in some sense a self-fulfilling prophecy. See chapter 8 of Technological Innovation in the Semiconductor Industry for a thorough discussion. ↩

16. Intel's 1985 Annual Report says "It was a miserable year for Intel" and discusses the decision to leave the DRAM business. ↩

Reverse engineering the 76477 "Space Invaders" sound effect chip from die photos

Remember the old video game Space Invaders? Some of its sound effects were provided by a chip called the 76477 Complex Sound Generation chip. While the sound effects1 produced by this 1978 chip seem primitive today, it was used in many video games, pinball games. But what's inside this chip and how does it work internally? By reverse-engineering the chip from die photos, we can find out. (Photos courtesy of Sean Riddle.) In this article, I explain how the analog circuits of this chip works and show how the hundreds of transistors on the silicon die form the circuits of this complex chip.

The 76477 chip combines several functional blocks to produce a variety of sound effects. A voltage-controlled oscillator (VCO) produces a signal whose frequency depends on the control voltage. A "super low frequency" SLF oscillator generates a triangle wave. Feeding this into the VCO generates a varying pitch, useful for bird chirps, sirens, or the warbling sound of the UFO in Space Invaders. A "one-shot" produces a pulse of a fixed length to control the length of the sound. An envelope generator makes the sound more realistic by ramping its amplitude (volume) up at the start and down at the end. A digital white noise generator can be used for drums, gunshots, explosions and other similar sound effects. Finally a digital mixer combines these signals and feeds them to the output amplifier.

The diagram below indicates the functional blocks on the 76477 die. Looking under a microscope, you can see the circuitry that makes up the chip. The yellowish lines are metal traces that connect the circuits of the die. The reddish and greenish regions are the silicon of the chip, forming transistors and resistors. The black blobs around the edges of the chip show where tiny bond wires connected the die to the integrated circuit pins. Analog circuits are outlined in purple, while digital circuits are in cyan. The 76477 is primarily analog—most control signals are analog, the chip has no digital registers, and most sounds are generated from analog circuits—but about a third of the chip's area is digital logic.2

Functionality blocks inside the 76477 sound chip, indicated on the die. Die photo courtesy of Sean Riddle.

The block diagram below shows the chip's functional elements and can be compared to the die photo above. The chip is primarily controlled by resistors (red pins), capacitors (cyan pins) and voltages (violet pins). This made the chip difficult to control with a microprocessor, and more useful for hardwired sounds.

Block diagram of the 76477 sound chip, from the datasheet. Resistor inputs: red, capacitor inputs: cyan, voltage inputs: violet.

The remainder of this article will dive into the internals of the 76477 chip. First I'll show how transistors and resistors are built on an integrated circuit. Next I'll explain two key analog building blocks: the current mirror and the comparator. Finally, I'll show the reverse-engineered circuits for the 76477's analog functional modules and discuss how they operate. (I'll describe the chip's digital logic in a future article.)

Integrated circuit transistors and resistors

A bipolar integrated circuit such as the 76477 is built from two types of transistors: NPN and PNP. The diagram below shows two transistors on the 76477 die, with the emitter, base and collector labeled. The N-doped silicon appears reddish, while P-doped silicon appears green. Metal lines (yellowish) on top of the silicon connect the circuits, with outlines visible where metal is connected to the silicon layer. The transistor on the left is an NPN transistor. Internally, the transistor is built vertically, with the emitter on top, the base forming a thin layer beneath the emitter, and the collector region underneath. The transistor on the right is a PNP transistor. The collector forms a ring surrounding the central emitter. 3

Two transistors as they appear on the die of the 76477, showing the Emitter, Base, and Collector.

Resistors are an important component of analog circuits. On a silicon chip, they can be formed from a long, narrow region of doped silicon with higher resistance. On an IC, resistors take a lot of space and are inaccurate, so they are generally avoided where possible. The die image below shows three resistors, which appear red in the photo. They are connected to the metal wiring at the contact points marked with blue arrows.

Three resistors (red) on the die of the 76477 chip. The ends of the resistors are connected to the metal layer at contact points marked in blue.

If a metal wire needs to cross another metal wire, the signal can use the silicon layer to pass under the wire. Two of these cross-unders are shown below. The silicon (green) is doped to be lower resistance than in the case of a resistor. Cross-unders are higher resistance than metal wiring, so they are only used when necessary.

A relatively low resistance silicon "wire" (green) passes under two metal wires.

By carefully examining the die photo, you can pick out the transistors and resistors and determine how they are connected. From this, you can reverse-engineer the chip's circuits.

The current mirror

A key component of most analog circuits is the current mirror, and the 76477 is no exception, containing many current mirrors. A current mirror takes one reference current and "clones" it, generating a current that matches the reference current. Either of the symbols below can be used to indicate a current mirror or current source.

Schematic symbols for a current source.

The following circuit shows the circuit for a current mirror with two current source outputs. A reference current passes through the transistor on the right. (In this case, the current is set by the resistor.) Since all the transistors have the same emitter voltage and base voltage, they source the same current, so the currents on the left match the reference current on the right.4

Current mirror circuit. The currents on the left copy the reference current on the right.

The die segment below shows four PNP current mirror transistors providing a dozen current outputs. Each of the three pinwheel-shaped transistors has four collectors surrounding the central emitter, allowing it to produce four matched current outputs. The lower left transistor is a standard PNP transistor with a ring-shaped collector. The large green rectangle in the center is the shared base connection for the transistors.

Four transistors from current mirrors in the 76477 sound chip. Three of them have four collectors surrounding the emitter, giving them a "pinwheel" appearance.

Current mirrors are commonly used to generate bias currents instead of pull-up resistors. Since resistors inside ICs are both inconveniently large and inaccurate, a current mirror is used when possible. If you look at the die image at the beginning of the article, note the large die area dedicated to current mirrors for bias current generation. 5

Comparators

Another key building block of the 76477 is the comparator, comparing two voltages and determining which one is higher. The heart of the comparator is a differential pair, a two-transistor circuit. If both inputs are equal, the transistors will conduct equally and the current will be split equally along both branches. But if one input is lower, that transistor will conduct more, switching most of the current into that branch.

The schematic below shows a typical comparator in the 76477. If the positive input is higher than the negative input, the comparator outputs a 1. Otherwise it outputs a 0. Transistors 3 and 4 form the differential pair. The current to them is supplied by a current mirror above them, and most of the current will be directed to the side with the lower input. Transistors 1 and 2 buffer the inputs (using emitter followers) and are biased by more current mirrors. Transistors 5 and 6 form another current mirror, used as an active load to double the circuit's amplification. Finally, transistors 7 and 8 form an inverter, generating a digital output from the comparator.

Schematic of comparator circuit in 76477 sound chip, slightly simplified.

The die image below shows one of the comparators used in the 76477 with the transistors labeled to match the schematic above. Note that transistors pairs 1 and 2, 3 and 4, and 5 and 6 have similar layouts to give them matched characteristics, improving the balance of the comparator. The base and collector of transistor 5 are connected together for the current mirror. The current sources and resistors are on another part of the die, not shown below.

Die image of the 76477 sound chip showing a comparator used in the one-shot circuit.

The one-shot

The one-shot is a simple circuit that generates one pulse of a set width , triggered when the chip's inhibit signal drops low. This pulse controls the duration of the sound. For instance, a short pulse of noise can be used for a gunshot sound, while a longer noise could be an explosion.

Schematic of one-shot circuit inside the 76477 sound chip.

The one-shot charges an external capacitor via an external resistor and current mirror. The resistor sets the reference current for the current mirror, and the mirror feeds this current into the capacitor. The advantage of using this charging circuit rather than a simple R-C circuit is that the charging current remains constant, rather than decreasing as the capacitor charges.7 This "charging trick" is used several times in the 76477.

When the capacitor's voltage reaches the comparator's limit level (2.6V), the comparator output goes low and the pulse ends. Thus, the faster the capacitor charges, the shorter the pulse. Digital logic circuitry (not shown) resets the one-shot by discharging the capacitor at the end of the pulse and holds it low until the next pulse is triggered via the inhibit pin.

The super-low frequency oscillator

The next functional block of the 76477 that I'll examine is the super-low frequency (SLF) oscillator. It generates a triangle wave that can control the voltage-controlled oscillator (VCO) to generate warbling sounds by ramping the pitch up and down. The frequency is controlled by an external resistor and an external capacitor. Like the one-shot, a current mirror provides a fixed charging current. However, the SLF oscillator uses a second current mirror to discharge the capacitor at the same rate, generating the triangle wave output.

Building a current mirror from NPN transistors creates a current mirror that sinks current instead of sourcing current, as in the lower current mirror. This current mirror also uses another trick: by using a transistor with two emitters, the current mirror doubles the output current; this is indicated on the schematic with two arrows in the current mirror circle. When the lower current mirror is disabled by the transistor, the capacitor is charged by the upper current mirror, with a current (I) set by the resistor, similar to the one-shot circuit. But with the lower current mirror enabled, the lower current mirror sinks current 2I. Since the upper current mirror is still supplying I to the capacitor, the net current (-I) discharges the capacitor. Thus, by combining two current mirrors, the capacitor can either be charged or discharged with the same current I. This trick will appear again in the VCO.

Schematic of SLF inside the 76477 sound chip.

The final piece of the SLF oscillator is the comparator. The + input is set to an upper limit of 2.46 volts, so the comparator will output 1 as the capacitor charges. When the capacitor reaches the limit voltage, the comparator output drops to 0. The first effect of this is to enable the lower mirror, so the capacitor starts discharging. The second effect is to pull the comparator input low (0.36V) through the hysteresis circuit.6 This keeps the comparator output low until the capacitor has discharged. Thus the circuit "remembers" if it is charging or discharging, without using a flip flop. The square wave output is used by the mixer and the triangle wave output is used by the VCO.

Zooming in on the die of the 76477 sound chip shows the circuitry for the SLF oscillator.

The diagram above shows how the SLF circuit looks on the die. Note the three transistors for the upper current mirror. Below the capacitor pin is a transistor for the lower current mirror with a doubled emitter; this causes the mirror's output current to be doubled. On the left are resistors forming the hysteresis circuit. The comparator circuit is underneath it. The Vcc power trace has been colored red and the ground trace has been colored blue.

The voltage-controlled oscillator (VCO)

The voltage-controlled oscillator generates a pitch that depends on its voltage input, as shown below. The circuit for the VCO has a lot in common with the SLF: it creates a triangle wave by charging and discharging an external capacitor. The main difference is that it charges until the capacitor reaches the control voltage (rather than a fixed voltage). Thus, the voltage input controls the pitch: with a higher control voltage, the capacitor takes longer to charge so the frequency is lower. This control voltage can be provided either from the SLF or an external pin. The "VCO select" pin selects which control voltage to use.

The triangle wave from the SLF oscillator can control the frequency of the Voltage Controlled Oscillator (VCO). From the 76477 datasheet.

The VCO's output is a digital square wave that is active during the charging half of the VCO's internal triangle wave. Another pin controls the output's duty cycle, the fraction of the time it is high.8 The triangle wave is compared to the duty cycle control voltage to determine when the output switches on and off. A lower control voltage results in a shorter duty cycle while a higher control voltage results in a longer duty cycle (up to 50%). The digital logic combines the two outputs to yield the final output. The VCO has two comparators in parallel; the VCO select input enables one of them.

Schematic of VCO inside the 76477 sound chip.

Envelope generation

The 76477 provides envelope generation, so the output can smoothly ramp up at the start of a sound and ramp down at the end, making it more realistic9 The diagram below (from the datasheet) shows the linear attack and decay applied to a sound waveform. (The sound below illustrates the random pulses produced by the white noise generator.)

A sound waveform with attack and decay applied.

The schematic below shows the circuit for envelope generation. As with the other circuits, a capacitor is charged and discharged using current mirrors, but two separate resistors are used so the charge (attack) and discharge (decay) rates can be different. The attack signal (from the digital logic) causes the envelope capacitor to charge through a current mirror at a rate controlled by an external attack resistor. The decay signal (simply the complement of the attack signal) causes the capacitor to discharge, controlled by the external decay resistor. Discharge uses a second current mirror operating as a current sink, but unlike earlier circuits it doesn't double the current. The inhibit signal rapidly discharges the capacitor, resetting the envelope.

Schematic of envelope generator inside the 76477 sound chip.

The output circuit

The 76477's output circuit uses four separate current mirrors. The varying reference current for the first current mirror is generated from the envelope voltage and the external amplitude resistor. Unlike the other control resistors, this resistor has a varying voltage applied so it produces a varying reference current. This current controls the output's overall amplitude.

Schematic of output circuit inside the 76477 sound chip.

The amplitude reference current goes into the second current mirror (lower left). The inhibit signal blocks this current mirror, which is how the inhibit signal blocks the chip's output. A third current mirror (upper right) generates two output currents referenced from the current sunk by the second current mirror. The final current mirror is enabled and disabled by the output signal from the mixer. Thus, the current to the op amp alternates between positive and negative, with magnitude depending on the envelope and the control resistor.

The output op amp drives triple-Darlington emitter-follower output transistors. These transistors are not particularly large, so the 76477 has limited output power. An external feedback resistor to the op amp controls the output's amplification. The die photo below shows part of the output circuit. A capacitor helps stabilize the output so it doesn't oscillate.

Part of the output circuit for the 76477 sound chip.

Conclusions

The 76477 is a complex integrated circuit with hundreds of transistors, but by examining the die the operation of the chip can be reverse engineered. The chip uses interesting techniques to generate sounds by combining oscillators, a noise generator and other functional blocks. Outside of analog chips, current mirrors are fairly obscure but the 76477 makes heavy use of current mirrors, with multiple current mirrors in almost every functional unit, driving comparators, generating bias currents, and providing uniform charge/discharge currents.

The chip had several disadvantages that led to its replacement by more advanced chips. The biggest inconvenience is that most of the 76477's parameters are controlled by resistors and capacitors, rather than digitally, making it hard to control the chip with a microprocessor. A second disadvantage of the chip is the sounds were largely digital square waves which gave the sounds a harsh quality rather than a "warm" analog sound. Finally, it was difficult to produce accurate pitches with the VCO, making the chip less useful for music synthesis. For these reasons, digitally-controlled chips such as the AY-3-8910 (1978) surpassed the 76477 in popularity.

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Thanks to Sean Riddle for the die photos. I'll end with his die photo of the 76477 after dissolving the metal layer in acid, making it easier to see the resistors and transistors.

Die photo of the 76477 sound chip. The metal layer has been dissolved with acid to reveal the silicon. Colors are enhanced. Photo courtesy of Sean Riddle.

Notes and references

1. The Space Invaders schematics show that the video game used seven different circuits to create its different sounds. The 76477 generated the "UFO" sound, while other sounds (saucer hit, explosion, missile, invader hit, etc.) were mostly generated by collections of op amps. ↩

2. The 76477's digital circuitry was built with Integrated Injection Logic (I2L), which was developed in the 1970s with the promise of building fast bipolar logic with the VLSI density of MOS logic. Spoiler: I2L lost out to CMOS, the technology used in microprocessors today. For the 76477, the most useful feature of I2L is that it used the same manufacturing process as analog bipolar transistors, allowing the analog and digital circuitry to be combined on one chip. In the 76477, digital logic is used for digitally selecting and combining audio signals, generating white noise with a nonlinear feedback shift register, and control functions. ↩

3. Some PNP transistors have a different structure; a PNP transistor with the collector grounded lacks the collector ring, using the grounded substrate as the collector. This makes it more similar to an NPN transistor both in appearance and construction. For details on the internal structure of bipolar IC transistors, see my earlier articles on the 555 and 741 chips. In brief, the NPN transistor is vertical in cross-section with the emitter on top and collector on the bottom. Most of the PNP transistors are lateral (i.e. horizontal) with the emitter on the inside and the collector on the outside, and the base in between (somewhat distant from the base connection). ↩

4. Many of the 76477's current mirrors are more complex, with a Darlington pair of transistors between the reference transistors's base and collector. This compensates for base currents and makes the mirror more accurate. See this paper for an explanation. ↩

5. For bias generation, you might wonder what provides the initial current to the current mirrors. The answer is an internal resistor, along with several transistors to keep the current stable. The circuit is similar to the self-biased current source described here. ↩

6. Note that the hysteresis circuit used by the SLF is not providing feedback like with an op amp. It provides a comparison voltage of 2.46V while charging and 0.36V while discharging. The SLF comparator's output is open collector, so it can only pull the circuit low. ↩

7. If you use a simple resistor-capacitor circuit instead of a current mirror, the capacitor charges more slowly as its voltage increases, resulting in an exponential charging curve. By using a current mirror, the current remains constant so the capacitor charges at a constant rate. The result is a linear triangle wave. ↩

8. The datasheet calls the VCO's duty cycle control a "pitch control". This is wrong, since the frequency is unaffected. ↩

9. Note that the envelope is the only analog part of the sound; until the envelope ramps are applied, the output is a digital square wave. ↩

Inside the vintage 74181 ALU chip: how it works and why it's so strange

The 74181 ALU (arithmetic/logic unit) chip powered many of the minicomputers of the 1970s: it provided fast 4-bit arithmetic and logic functions, and could be combined to handle larger words, making it a key part of many CPUs. But if you look at the chip more closely, there are a few mysteries. It implements addition, subtraction, and the Boolean functions you'd expect, but why does it provide several bizarre functions such as "A plus (A and not B)"? And if you look at the circuit diagram (below), why does it look like a random pile of gates rather than being built from standard full adder circuits. In this article, I explain that the 74181's set of functions isn't arbitrary but has a logical explanation. And I show how the 74181 implements carry lookahead for high speed, resulting in its complex gate structure.

Schematic of the 74LS181 ALU chip, from the datasheet. The internal structure of the chip is surprisingly complex and difficult to understand at first.

The 74181 chip is important because of its key role in minicomputer history. Before the microprocessor era, minicomputers built their processors from boards of individual chips. A key part of the processor was the arithmetic/logic unit (ALU), which performed arithmetic operations (addition, subtraction) and logical operations (AND, OR, XOR). Early minicomputers built ALUs out of a large number of simple gates. But in March 1970, Texas Instruments introduced the 74181 Arithmetic / Logic Unit (ALU) chip, which put a full 4-bit ALU on one fast TTL chip. This chip provided 32 arithmetic and logic functions, as well as carry lookahead for high performance. Using the 74181 chip simplified the design of a minicomputer processor and made it more compact, so it was used in many minicomputers. Computers using the 74181 ranged from the popular PDP-11 and Xerox Alto minicomputers to the powerful VAX-11/780 "superminicomputer". The 74181 is still used today in retro hacker projects.1

The 74181 implements a 4-bit ALU providing 16 logic functions and 16 arithmetic functions, as the datasheet (below) shows. As well as the expected addition, subtraction, and Boolean operations, there are some bizarre functions such as "(A + B) PLUS AB".

The datasheet for the 74181 ALU chip shows a strange variety of operations.

So how is the 74181 implemented and why does it include such strange operations? Is there any reason behind the 74181's operations, or did they just randomly throw things in? And why are the logic functions and arithmetic functions in any particular row apparently unrelated? I investigated the chip to find out.

The 16 Boolean logic functions

There's actually a system behind the 74181's set of functions: the logic functions are the 16 possible Boolean functions f(A,B). Why are there 16 possible functions? If you have a Boolean function f(A,B) on one-bit inputs, there are 4 rows in the truth table. Each row can output 0 or 1. So there are 2^4 = 16 possible functions. Extend these to 4 bits, and these are exactly the 16 logic functions of the 74181, from trivial 0 and 1 to expected logic like A AND B to contrived operations like NOT A AND B. These 16 functions are selected by the S0-S3 select inputs.

Arithmetic functions

The 74181's arithmetic operations are a combination of addition, subtraction, logic operations, and strange combinations such as "A PLUS AB PLUS 1". It turns out that there is a rational system behind the operation set: they are simply the 16 logic functions added to A along with the carry-in.2 That is, the arithmetic functions are: A PLUS f(A,B) PLUS carry-in. For example, If f(A,B)=B, you get simple addition: A PLUS B PLUS carry-in. If f(A,B) = NOT B, you get A PLUS NOT B PLUS carry-in, which in two's-complement logic turns into subtraction: A MINUS B MINUS 1 PLUS carry-in.

Other arithmetic functions take a bit more analysis. Suppose f(A,B) = NOT (A OR B). Then each bit of A PLUS f(A,B) will always be 1 except in the case where A is 0 and B is 1, so the result of the sum is A OR NOT B. Even though you're doing addition, the result is a logical function since no carry can be generated. The other strange arithmetic functions can be understood similarly.3

Thus, the 16 arithmetic functions of the 74181 are a consequence of combining addition with one of the 16 Boolean functions. Even though many of the functions are strange and probably useless, there's a reason for them. (The Boolean logic functions for arithmetic are in a different order than for logical operations, explaining why there's no obvious connection between the arithmetic and logical functions.)

Carry lookahead: how to do fast binary addition

The straightforward but slow way to build an adder is to use a simple one-bit full adders for each bit, with the carry out of one adder going into the next adder. The result is kind of like doing long addition by hand: in decimal if you add 9999 + 1, you have to carry the 1 from each column to the next, which is slow. This "ripple carry" makes addition a serial operation instead of a parallel operation, harming the processor's performance. To avoid this, the 74181 computes the carries first and then adds all four bits in parallel, avoiding the delay of ripple carry. This may seem impossible: how can you determine if there's a carry before you do the addition? The answer is carry lookahead.

Carry lookahead uses "Generate" and "Propagate" signals to determine if each bit position will always generate a carry or can potentially generate a carry. For instance, if you're adding 0+0+C (where C is the carry-in), there's no way to get a carry out from that addition, regardless of what C is. On the other hand, if you're adding 1+1+C, there will always be a carry out generated, regardless of C. This is called the Generate case. Finally, for 0+1+C (or 1+0+C), there will be a carry out if there is a carry in. This is called the Propagate case since if there is a carry-in, it is propagated to the carry out.4 Putting this all together, for each bit position you create a G (generate) signal if both bits are 1, and a P (propagate) signal unless both bits are 0.

The carry from each bit position can be computed from the P and G signals by determining which combinations can produce a carry. For instance, there will be a carry from bit 0 to bit 1 if P₀ is set (i.e. a carry is generated or propagated) and there is either a carry-in or a generated carry. So C₁ = P₀ AND (C_in OR G₀).

Higher-order carries have more cases and are progressively more complicated. For example, consider the carry in to bit 2. First, P₁ must be set for a carry out from bit 1. In addition, a carry either was generated by bit 1 or propagated from bit 0. Finally, the first carry must have come from somewhere: either carry-in, generated from bit 0 or generated from bit 1. Putting this all together produces the function used by the 74181: C₂ = P₁ AND (G₁ OR P₀) AND (C₀ OR G₀ OR G₁).

As you can see, the carry logic gets more complicated for higher-order bits, but the point is that each carry can be computed from G and P terms and the carry-in. Thus, the carries can be computed in parallel, before the addition takes place.5

Creating P and G with an arbitrary Boolean function

The previous section showed how the P (propagate) and G (generate) signals can be used when adding two values. The next step is to examine how P and G are created when adding an arbitrary Boolean function f(A, B), as in the 74181. The table below shows P and G when computing "A PLUS f(A,B)". For instance, when A=0 there can't be a Generate, and Propagate depends on the value of f. And when A=1, there must be a Propagate, while Generate depends on the value of f.

A	B	A PLUS f(a,b)	P	G
0	0	0+f(0,0)	f(0,0)	0
0	1	0+f(0,1)	f(0,1)	0
1	0	1+f(1,0)	1	f(1,0)
1	1	1+f(1,1)	1	f(1,1)

In the 74181, the four f values are supplied directly by the four Select (S pin) values, resulting in the following table:6

A	B	A PLUS f	P	G
0	0	0	S1	0
0	1	1	S0	0
1	0	1	1	S2
1	1	10	1	S3

The chip uses the logic block below (repeated four times) to compute P and G for each bit. It is straightforward to verify that it implements the table above. For instance, G will be set if A is 1, B is 1 and S3 is 1, or if A is 1, B is 0 and S2 is set.

This circuit computes the G (generate) and P (propagate) signals for each bit of the 74181 ALU chip's sum. The S0-S3 selection lines select which function is added to A.

Creating the arithmetic outputs

The addition outputs are generated from the internal carries (C0 through C3), combined with the P and G signals. For each bit, A PLUS f is the same as P ⊕ G, so adding in the carry gives us the full 4-bit sum. Thus, F0 = C0 ⊕ P0 ⊕ G0, and similarly for the other F outputs.7 On the schematic, each output bit has two XOR gates for this computation.

Creating the logic outputs

For the logic operations, the carries are disabled by forcing them all to 1. To select a logic operation, the M input is set to 1. M is fed into all the carry computation's AND-NOR gates, forcing the carries to 1. The output bit sum as as above, producing A ⊕ f ⊕ 1 = A ⊕ f. This expression yields all 16 Boolean functions, but in a scrambled order relative to the arithmetic functions.8

Interactive 74181 viewer

To see how the circuits of the 74181 work together, try the interactive schematic below.9 The chip's inputs are along the top and right; click on any of them to change the value. The A and B signals are the two 4-bit arguments. The S bits on the right select the operation. C is the carry-in (which is inverted). M is the mode, 1 for logic operations and 0 for arithmetic operations. The dynamic chart under the schematic describes what operation is being performed.

The P and G signals are generated by the top part of the circuitry, as described above. Below this, the carry lookahead logic creates the carry (C) signals by combining the P and G signals with the carry-in (Cn). Finally, the sum for each bit is generated (Σ) from the P and G signals7, then combined with each carry to generate the F outputs in parallel.10

Result and truth table for inputs entered above
Select :

	0000
A	0000
B	0001
F	1001

Ai	Bi	Pi	Gi	Fi
0	0	X	Y	F
0	1	X	Y	F
1	0	X	Y	F
1	1	X	Y	F

Die photo of the 74181 chip.

I opened up a 74181, took die photos, and reverse engineered its TTL circuitry. My earlier article discusses the circuitry in detail, but I'll include a die photo here since it's a pretty chip. (Click image for full size.) Around the edges you can see the thin bond wires that connect the pads on the die to the external pins. The shiny golden regions are the metal layer, providing the chip's internal wiring. Underneath the metal, the purplish silicon is doped to form the transistors and resistors of the TTL circuits. The die layout closely matches the simulator schematic above, with inputs at the top and outputs at the bottom.

Die photo of the 74181 ALU chip. The metal layer of the die is visible; the silicon (forming transistors and resistors) is hidden behind it.

Conclusion

While the 74181 appears at first to be a bunch of gates randomly thrown together to yield bizarre functions, studying it shows that there is a system to its function set: it provides all 16 Boolean logic functions, as well as addition to these functions. The circuitry is designed around carry lookahead, generating G and P signals, so the result can be produced in parallel without waiting for carry propagation. Modern processors continue to use carry lookahead, but in more complex forms optimized for long words and efficient chip layout.12

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Notes and references

1. Retro projects using the 74181 include the APOLLO181 CPU, Fourbit CPU, 4 Bit TTL CPU, Magic-1 (using the 74F381), TREX, Mark 1 FORTH and Big Mess o' Wires. ↩

2. The carry-in input and the carry-out output let you chain together multiple 74181 chips to add longer words. The simple solution is to ripple the carry from one chip to the next, and many minicomputers used this approach. A faster technique is to use a chip, the 74182 look-ahead carry generator, that performs carry lookahead across multiple 74181 chips, allowing them to all work in parallel. ↩

3. One thing to note is A PLUS A gives you left shift, but there's no way to do right shift on the 74181 without additional circuitry. ↩

4. To simplify the logic, the 74181 considers 1+1+C both a Propagate case and a Generate case. (Some carry lookahead systems consider 1+1+C to be a Propagate case but not a Generate case.) For the 74181's outputs, Propagate must be set for Generate to be meaningful. ↩

5. The carry-lookahead logic in the 74181 is almost identical to the earlier 74LS83 adder chip. The 74181's circuitry can be viewed as an extension of the 74LS83 to support 16 Boolean functions and to support logical functions by disabling the carry. ↩

6. The way the S0 and S1 values appear in the truth table seems backwards to me, but that's how the chip works. ↩

7. The bit sum Σ can be easily produced from the P and G signals. Some datasheets show each Σ signal generated as P XOR G, while other datasheets show Σ generated as NOT P AND G. Since the combination P=0, G=1 never arises, both generate the same results. I show XOR on the schematic as it is conceptually easier to understand, but examining the die shows the physical circuit uses the NOT/AND gates. ↩

8. The logic functions are defined in terms of Select inputs as follows:

   A	B	F
   0	0	S1
   0	0	S0
   0	0	S2
   0	0	S3

   Because the first two terms are inverted, the logic function for a particular select input doesn't match the arithmetic function. ↩

9. The schematic is based on a diagram by Poil on WikiMedia, CC By-SA 3.0, with circuitry and labeling changes. ↩

10. The 74181 chip has a few additional outputs. The A=B output is used with the subtraction operation to test the two inputs for equality. The Cn+4 output is the inverted carry out, supporting longer words. P and G are the carry propagate and generate outputs, used for carry lookahead with longer words.11 ↩

11. The P and G outputs in my schematic are reversed compared to the datasheet, for slightly complicated reasons. I'm describing the 74181 with active-high logic, where a high signal indicates 1, as you'd expect. However, the 74181 can also be used with active-low logic, where a low signal indicates a 1. The 74181 works fine with active-low logic except the meanings of some pins change, and the operations are shuffled around. The P and G labels on the datasheet are for active-low logic, so with active-high, they are reversed. ↩

12. One example of a modern carry lookahead adder is Kogge-Stone. See this presentation for more information on modern adders, or this thesis for extensive details. ↩