How Hacker News ranking really works: scoring, controversy, and penalties

Update (11/18): article on penalties is penalized

Ironically, this article was penalized on Hacker News. Minutes after reaching the front page, a heavy 0.2 penalty was applied to the article, forcing it off the front page. The black line in the graph below shows the position of this article on Hacker News. You can see the sharp drop when the penalty was applied. The gray line shows where the article would have been ranked without the penalty. Without the penalty, the article would have been in the #5 spot, but with the penalty it never made it back onto the front page (positions 1-30). The lower green line shows the raw score of this article. (11/26: I'm told that the penalty was because the "voting ring detection" triggered erroneously.)

This article was penalized shortly after reaching the front page of Hacker News

The basic formula for Hacker News ranking has been known for years, but questions remained. Does the published code give the real algorithm? Are rankings purely based on votes or do invisible factors come into play? Do stories about the NSA get pushed down in the rankings? Why did that popular story suddenly disappear from the front page after you commented on it?

By carefully analyzing the top 60 HN stories for several days, I can answer those questions and more. The published formula is mostly accurate. There is much more tweaking of rankings than you'd expect, with 20% of front-page stories getting penalized in various ways. Anything with "NSA" in the title is penalized and drops off quickly. A "controversial" story gets severely penalized after hitting 40 comments. This article describes scoring and penalties in detail.

How ranking works

Articles are scored based on their upvote score, the time since the article was submitted, and various penalties using the following formula:

score=\frac{ \left( votes-1 \right) ^{.8}}{ \left( age _{hours} + 2 \right) ^ {1.8}} * penalties

Because the time has a larger exponent than the votes, an article's score will eventually drop to zero, so nothing stays on the front page too long. This exponent is known as gravity.

You might expect that every time you visit Hacker News, the stories are scored by the above formula and sorted to determine their rankings. But for efficiency, stories are individually reranked only occasionally. When a story is upvoted, it is reranked and moved up or down the list to its appropriate spot, leaving the other stories unchanged. Thus, the amount of reranking is significantly reduced. There is, however, the possibility that a story stops getting votes and ends up stuck in a high position. To avoid this, every 30 seconds one of the top 50 stories is randomly selected and reranked. The consequence is that a story may be "wrongly" ranked for many minutes if it isn't getting votes. In addition, pages can be cached for 90 seconds.

Raw scores and the #1 spot on a typical day

The following image shows the raw scores (excluding penalties) for the top 60 HN articles throughout the day of November 11. Each line corresponds to an article, colored according to its position on the page. The red line shows the top article on HN. Note that because of penalties, the article with the top raw score often isn't the top article.

Hacker News raw article scores throughout a day. Red line indicates the #1 article. Due to penalties, the #1 article does not always have the top score.

This chart shows a few interesting things. The score for an article shoots up rapidly and then slowly drops over many hours. The scoring formula accounts for much of this: an article getting a constant rate of votes will peak quickly and then gradually descend. But the observed peak is even faster - this is because articles tend to get a lot of votes in the first hour or two, and then the voting rate drops off. Combining these two factors yields the steep curves shown.

There are a few articles each day that score much above the rest, along with a lot of articles in the middle. Some articles score very well but are unlucky and get stuck behind a more popular article. Other articles hit #1 briefly, between the fall of one and the climb of another.

Looking at the difference between the article with the top raw score (top of the graph) and the top-ranked article (red line), you can see when penalties have been applied. The article Getting website registration completely wrong hit #1 early in the morning, but was penalized for controversy and rapidly dropped down the page, letting Linux ate my RAM briefly get the #1 spot before Simpsons in CSS overtook it. A bit later, the controversy penalty was applied to Apple Maps shortly after it reached the #1 spot, causing it to lose its #1 spot and rapidly drop down the rankings. The Snapchat article reached the top of HN but was penalized so heavily at 8:22 am that it dropped off the chart entirely. Why you should never use MongoDB was hugely popular and would have spent much of the day in the #1 spot, except it was rapidly penalized and languished around #7. Severing ties with the NSA started off with a NSA penalty but was so hugely popular it still got the #1 spot. However, it was quickly given an even bigger penalty, forcing it down the page. Finally, near the end of the day $4.1m goes missing was penalized. As it turns out, it would have soon lost the #1 spot to FTL even without the penalty.

The green triangles and text show where "controversy" penalties were applied. The blue triangles and text show where articles were penalized into oblivion, dropping off the top 60. Milder penalties are not shown here.

It's clear that the content of the #1 spot on HN isn't "natural", but results from the constant application of penalties to many articles. It's unclear if these penalties result from HN administrators or from flagged articles.

Submissions that get automatically penalized

Some submissions get automatically penalized based on the title, and others get penalized based on the domain. It appears that any article with NSA in the title gets an automatic penalty of .4. I looked for other words causing automatic penalties, such as awesome, bitcoin, and bubble but they do not seem to get penalized.

I observed that many websites appear to automatically get a penalty of .25 to .8: arstechnica.com, businessinsider.com, easypost.com, github.com, imgur.com, medium.com, quora.com, qz.com, reddit.com, rt.com, stackexchange.com, theguardian.com, theregister.com, theverge.com, torrentfreak.com, youtube.com. I'm sure the actual list is longer. (This is separate from "banned" sites, which were listed at one point.

One interesting theory by eterm is that news from popular sources gets submitted in parallel by multiple people resulting in more upvotes than the article "merits". Automatically penalizing popular websites would help counteract this effect.

The impact of penalties

Using the scoring formula, the impact of a penalty can be computed. If an article gets a penalty factor of .4, this is equivalent to each vote only counting as .3 votes. Alternatively, the article will drop in ranking 66% faster than normal. A penalty factor of .1 corresponds to each vote counting as .05 votes, or the article dropping at 3.6 times the normal rate. Thus, a penalty factor of .4 has a significant impact, and .1 is very severe.

Controversy

In order to prevent flamewars on Hacker News, articles with "too many" comments will get heavily penalized as "controversial". In the published code, the contro-factor function kicks in for any post with more than 20 comments and more comments than upvotes. Such an article is scaled by (votes/comments)^2. However, the actual formula is different - it is active for any post with more comments than upvotes and at least 40 comments. Based on empirical data, I suspect the exponent is 3, rather than 2 but haven't proven this.

The controversy penalty can have a sudden and catastrophic effect on an article's ranking, causing an article to be ranked highly one minute and vanish when it hits 40 comments. If you've wondered why a popular article suddenly vanishes from the front page, controversy is a likely cause. For example, Why the Chromebook pundits are out of touch with reality dropped from #5 to #22 the moment it hit 40 comments, and Show HN: Get your health records from any doctor' was at #17 but vanished from the top 60 entirely on hitting 40 comments.

My methodology

I crawled the /news and /news2 pages every minute (staying under the 2 pages per minute guideline). I parsed the (somewhat ugly) HTML with Beautiful Soup, processed the results with a big pile of Python scripts, and graphed results with the incomprehensible but powerful matplotlib.

The basic idea behind the analysis is to generate raw scores using the formula and then look for anomalies. At a point in time (e.g. 11/09 8:46), we can compute the raw scores on the top 10 stories:

2.802 Pyret: A new programming language from the creators of Racket
1.407 The Big Data Brain Drain: Why Science is in Trouble
1.649 The NY Times endorsed a secretive trade agreement that the public can't read
0.785 S.F. programmers build alternative to HealthCare.gov (warning: autoplay video)
0.844 Marelle: logic programming for devops
0.738 Sprite Lamp
0.714 Why Teenagers Are Fleeing Facebook
0.659 NodeKnockout is in Full Tilt. Checkout some demos
0.805 ISO 1
0.483 Shopify accepts Bitcoin.
0.452 Show HN: Understand closures
Note that three of the top 10 articles are ranked lower than expected from their score: The NY Times, Marelle and ISO 1. Since The NY Times is ranked between articles with 1.407 and 0.785, its penalty factor can be computed as between .47 and .85. Likewise, the other penalties must be .87 to .93, and .60 to .82.

I observed that most stories are ranked according to their score, and the exceptions are consistently ranked much lower, indicating a penalty. This indicates that the scoring formula in use matches the published code. If the formula were different, for instance the gravity exponent were larger, I'd expect to see stories drift out of their "expected" ranking as their votes or age increased, but I never saw this.

This technique shows the existence of a penalty and gives a range for the penalty, but determining the exact penalty is difficult. You can look at the range over time and hope that it converges to a single value. However, several sources of error mess this up. First, the neighboring articles may also have penalties applied, or be scored differently (e.g. job postings). Second, because articles are not constantly reranked, an article may be out of place temporarily. Third, the penalty on an article may change over time. Fourth, the reported vote count may differ from the actual vote count because "bad" votes get suppressed. The result is that I've been able to determine approximate penalties, but there is a fair bit of numerical instability.

Penalties over a day

The following graph shows the calculated penalties over the course of a day. Each line shows a particular article. It should start off at 1 (no penalty), and then drop to a penalty level when a penalty is applied. The line ends when the article drops off the top 60, which can be fairly soon after the penalty is applied. There seem to be penalties of 0.2 and 0.4, as well as a lot in the 0.8-0.9 range. It looks like a lot of penalties are applied at 9am (when moderators arrive?), with more throughout the day. I'm experimenting with different algorithms to improve the graph since it is pretty noisy.

On average, about 20% of the articles on the front page have been penalized, while 38% of the articles on the second page have been penalized. (The front page rate is lower since penalized articles are less likely to be on the front page, kind of by definition.) There is a lot more penalization going on than you might expect.

Here's a list of the articles on the front page on 11/11 that were penalized. (This excludes articles that would have been there if they weren't penalized.) This list is much longer than I expected; scroll for the full list.

Why the Climate Corporation Sold Itself to Monsanto, Facebook Publications, Bill Gates: What I Learned in the Fight Against Polio, McCain says NSA chief Keith Alexander 'should resign or be fired', You are not a software engineer, What is a y-combinator?, Typhoon Haiyan kills 10,000 in Philippines, To Persuade People, Tell Them a Story, Tetris and The Power Of CSS, Microsoft Research Publications, Moscow subway sells free tickets for 30 sit-ups, The secret world of cargo ships, These weeks in Rust, Empty-Stomach Intelligence, Getting website registration completely wrong, The Six Most Common Species Of Code, Amazon to Begin Sunday Deliveries, With Post Office's Help, Linux ate my RAM, Simpsons in CSS, Apple maps: how Google lost when everyone thought it had won, Docker and Go: why did we decide to write Docker in Go?, Amazon Code Ninjas, Last Doolittle Raiders make final toast, Linux Voice - A new Linux magazine that gives back, Want to download anime? Just made a program for that, Commit 15 minutes to explain to a stranger why you love your job., Why You Should Never Use MongoDB, Show HN: SketchDeck - build slides faster, Zero to Peanut Butter Docker Time in 78 Seconds, NSA's Surveillance Powers Extend Far Beyond Counterterrorism, How Sentry's Open Source Service Was Born, Real World OCaml, Show HN: Get your health records from any doctor, Why the Chromebook pundits are out of touch with reality, Towards a More Modular Future for JavaScript Libraries, Why is virt-builder written in OCaml?, IOS: End of an Era, The craziest things you can plug into your iPhone's audio jack, RFC: Replace Java with Go in default languages, Show HN: Find your health plan on Health Sherpa, Web Latency Benchmark: A new kind of browser benchmark, Why are Amazon, Facebook and Yahoo copying Microsoft's stack ranking system?, Severing Ties with the NSA, Doctor performs surgery using Google Glass, Duplicity + S3: Easy, cheap, encrypted, automated full-disk backups, Bitcoin's UK future looks bleak, Amazon Redshift's New Features, You're only getting the nice feedback, Software is Easy, Hardware is of Medium Difficulty, Facebook Warns Users After Adobe Breach, International Space Station Infected With USB Stick Malware, Tidbit: Client-Side Bitcoin Mining, Go: "I have already used the name for *MY* programming language", Multi-Modal Drone: Fly, Swim & Drive, The Daily Go Programming Newspaper, "We have no food, we need water and other things to survive.", Introducing the Humble Store, The Six Most Common Species Of Code, $4.1m goes missing as Chinese bitcoin trading platform GBL vanishes, Could Bitcoin Be More Disruptive than the Internet?, Apple Store is updating.

The code for the scoring formula

The Arc source code for a version of the HN server is available, as well as an updated scoring formula:
  (= gravity* 1.8 timebase* 120 front-threshold* 1
       nourl-factor* .4 lightweight-factor* .17 gag-factor* .1)

    (def frontpage-rank (s (o scorefn realscore) (o gravity gravity*))
      (* (/ (let base (- (scorefn s) 1)
              (if (> base 0) (expt base .8) base))
            (expt (/ (+ (item-age s) timebase*) 60) gravity))
         (if (no (in s!type 'story 'poll))  .8
             (blank s!url)                  nourl-factor*
             (mem 'bury s!keys)             .001
                                            (* (contro-factor s)
                                               (if (mem 'gag s!keys)
                                                    gag-factor*
                                                   (lightweight s)
                                                    lightweight-factor*
                                                   1)))))
In case you don't read Arc code, the above snippet defines several constants: gravity* = 1.8, timebase* = 120 (minutes), etc. It then defines a method frontpage-rank that ranks a story s based on its upvotes (realscore) and age in minutes (item-age).

The penalty factor is defined by an if with several cases. If the article is not a 'story' or 'poll', the penalty factor is .8. Otherwise, if the URL field is blank (Ask HN, etc.) the factor is nourl-factor*. If the story has been flagged as 'bury', the scale factor is 0.001 and the article is ranked into oblivion. Finally, the default case combines the controversy factor and the gag/lightweight factor. The controversy factor contro-factor is intended to suppress articles that are leading to flamewars, and is discussed more later.

The next factor hits an article flagged as a gag (joke) with a heavy value of .1, and a "lightweight" article with a factor of .17. The actual penalty system appears to be much more complex than what appears in the published code.

Conclusion

An article's position on the Hacker News home page isn't the meritocracy based on upvotes that you might expect. By carefully examining the articles that appear on the Hacker News page, we can learn a great deal about the scoring formula in use. While upvotes are the obvious factor controlling rankings, there is also a complex "penalty" system causing articles to be ranked lower or disappear entirely. This isn't just preventing spam, but affects many very popular articles. And if an article has more comments than votes, don't add your comment to it or you may kill it off entirely!

See discussion on Hacker News.

The Z-80's 16-bit increment/decrement circuit reverse engineered

The 8-bit Z-80 processor was very popular in the late 1970s and early 1980s, powering many personal computers such as the Osborne 1, TRS-80, and Sinclair ZX Spectrum. It has a 16-bit incrementer/decrementer that efficiently updates the program counter and stack pointer, as well as supporting several 16-bit instructions and memory refresh. By reverse engineering detailed die photographs of the Z-80, we can see exactly how this increment/decrement circuit works and discover the interesting optimizations it uses for efficiency.

The Z-80 microprocessor die, showing the main components of the chip.

The Z-80 microprocessor die, showing the main components of the chip.

The increment/decrement circuit in the lower left corner of the chip photograph above. This circuit takes up a significant amount of space on the chip, illustrating its complexity. It is located close to the register file, allowing it to access the registers directly.

The fundamental use for an incrementer is to step the program counter from instruction to instruction as the program executes. Since this happens at least once for every instruction, a fast incrementer is critical to the performance of the chip. For this reason, the incrementer/decrementer is positioned close to the address pins (along the left and bottom of the photograph above). A second key use is to decrement the stack pointer as data is pushed to the stack, and increment the stack pointer as data is popped from the stack. (This may seem backwards, but the stack grows downwards so it is decremented as data is pushed to the stack.)

The incrementer/decrementer in the Z-80 is also used for a variety of other instructions. For example, the INC and DEC instructions allow 16-bit register pairs to be incremented and decremented. The Z-80 includes powerful block copy and compare instructions (LDIR, LDDR, CPIR, CPDR) that can process up to 64K bytes with a single instruction. These instructions use the 16-bit BC register pair as a loop counter, and the decrementer updates this register pair to count the iterations.

One of the innovative features of the Z-80 is that it includes a DRAM refresh feature. Because Dynamic RAM (DRAM) stores data in capacitors instead of flip flops, the data will drain away if not accessed and refreshed every few milliseconds. Early microcomputer memory boards required special refresh hardware to periodically step through the address space and refresh memory. The incrementer is used to update the address in the refresh register R on each instruction. (Current systems still require memory refresh, but it is handled by the DDR memory modules and memory controller).

Architecture

The architecture diagram below provides a simplified view of how the incrementer/decrementer works with the rest of the Z-80. The incrementer is closely associated with the 16-bit address bus. The data bus, on the other hand, is only 8 bits wide. Many of the registers are 8 bits, but can be paired together as 16-bit registers (BC, DE, HL).

A 16-bit latch feeds into the incrementer. This is needed since if a value were read from the PC, incremented, and written back to the PC at the same time a loop would occur. By latching the value, the read and write are done in separate cycles, avoiding instability. On the chip, the latch is between the incrementer and the register file.

The program counter and refresh register are separated from the rest of the registers and coupled closely to the incrementer. This allows the incrementer to be used in parallel with the rest of the Z-80. In particular, for each instruction fetch, the program counter (PC) is written to the address bus and incremented. Then the refresh address is written to the address bus for the refresh cycle, and the R register is incremented. (Note that the interrupt vector register I is in the same register pair as the R register. This explains why the I value is also written to the address bus during refresh.)

This diagram shows how the incrementer/decrementer is used in the Z-80 microprocessor.

This diagram shows how the incrementer/decrementer is used in the Z-80 microprocessor.

One of the interesting features of the Z-80 is a limited form of pipelining: fetch/execute overlap. Usually, the Z-80 fetches an instruction before the previous instruction has finished executing. The architecture above shows how this is possible. Because the PC and R registers are separated from the other registers, the other registers and ALU can continue to operate during the fetch and refresh steps.

The other registers are not entirely separated from the incrementer/decrementer, though. The stack pointer and other registers can communicate via the bus with the incrementer/decrementer when needed. Pass transistors allow this bus connection to be made as needed.

How a simple incrementer/decrementer works

To understand the circuit, it helps to start with a simple incrementer. If you've studied digital circuits, you've probably seen how two bits can be added with a half-adder, and several half-adders can be chained together to implement a simple multi-bit increment circuit.

The circuit below shows a half-adder, which can increment a single bit. The sum of two bits is computed by XOR, and if both bits are 1, there is a carry.

A simple half-adder that can be used to build an incrementer.

A simple half-adder that can be used to build an incrementer.

Chaining together 16 of these half-adder circuits creates a 16-bit incrementer. Each carry-out is connected to the carry-in of the next bit. A 1 value is fed into the initial carry-in to start the incrementing.

This circuit can be converted to a decrement circuit by renaming the carry signal as a borrow signal. If a bit is 0 and borrow is 1, then there must be a borrow from the next higher bit. (This is similar to grade-school decimal subtraction: 101000 - 1 = 100999 in decimal, since you keep borrowing until you hit a nonzero digit.) When decrementing, a 0 bit potentially causes a borrow, the opposite of incrementing, where a 1 bit potentially causes a carry.

The incrementer and decrementer can be combined into a single circuit by adding one more gate. When computing the carry/borrow for decrementing, each bit is flipped. This is accomplished by using an XOR gate with the decrement condition as an input. If decrement is 1, the input bit is flipped. To increment, the decrement input is set to 0 and the bit passes through the XOR gate unchanged.

A half-adder / subtractor that can be used to build an incrementer/decrementer.

A half-adder / subtractor that can be used to build an incrementer/decrementer.

Repeating the above circuit 16 times creates a 16-bit incrementer/decrementer.

Ripple carry: the problem and solutions

While the circuits above are simple, they have a big problem: they are slow. These circuits use what is called "ripple carry", since the carry value ripples through the circuit bit by bit. The consequence is each bit can't be computed until the carry/borrow is available from the previous bit. This propagation delay limits the clock speed of the system, since the final result isn't available until the carry has made it way through the entire circuit. For a 16-bit counter, this delay is significant.

Carry skip

The Z-80 uses two techniques to avoid ripple carry and speed up the incrementer. First, it uses a technique called carry-skip to compute the result and carry for two bits at a time, reducing the propagation delay.

The circuit diagram below shows how two bits at a time can be computed. Both carry values are computed in parallel, rather than the second carry depending on the first. If both input bits are 1 and there is a carry in, then there is a carry from the left bit. By computing this directly, the propagation delay is reduced.

A circuit to increment or decrement two bits at once.

A circuit to increment or decrement two bits at once.

Due to the MOS gates used in the Z-80, NOR and XNOR gates are more practical than AND and XOR gates, so instead of the carry skip circuit above, the similar circuit below is used in the Z-80. The output bits are inverted, but this is not a problem because many of the Z-80's internal buses are inverted. (The Z-80 uses an interesting pass-transistor XNOR gate, described here. The circuit below performs increment/decrement on two bits, and is repeated six times in the Z-80. To simplify the final schematic, the circuit in the dotted box will simply be shown as a box labeled "2-bit inc/dec".

The circuit used in the Z-80 to increment or decrement two bits.

The circuit used in the Z-80 to increment or decrement two bits.

Carry-lookahead

The second technique used by the Z-80 to avoid the ripple carry delay is carry lookahead, which computes some of the carry values directly from the inputs without waiting on the previous carries. If a sequence of bits is all 1's, there will be a carry from the sequence when it is incremented. Conversely, if there is a 0 anywhere in the sequence, any intermediate carry will be "extinguished". (Similarly, all 0's causes a borrow when decrementing.) By feeding the bits into an AND gate, a sequence of all 1's can be detected, and the carry immediately generated. (The Z-80 uses the inverted bits and a NOR gate, but the idea is the same.)

In the Z-80 three lookahead carries are computed. The carry from the lowest 7 bits is computed directly. If these bits are all 1, and there is a carry-in, then there will be a carry out. The second carry lookahead checks bits 7 through 11 in parallel. The third carry lookahead checks bits 12 through 14 in parallel. Thus, the last bit of the result (bit 15) depends on three carry lookahead steps, rather than 15 ripple steps. This reduces the time for the incrementer to complete.

For more information on carry optimization, see this or this discussion of adders.

The Z-80's increment/decrement circuit

The schematic below shows the actual circuit used in the Z-80 to implement the 16-bit incrementer/decrementer, as determined by reverse engineering the silicon. It uses six of the 2-bit inc/dec blocks described earlier in combination with the three carry-lookahead gates.

In the top half of the schematic, the seven low-order bits are incremented/decremented using the circuit block discussed above. In parallel, the carry/borrow from these bits is computed by the large NOR gate on the left.

Bits 7 through 11 are computed using the carry lookahead value, allowing them to be computed without waiting on the low-order bits. In parallel, the carry/borrow out of these bits is computed by the large NOR gate in the middle, and used to compute bits 12 through 14. The last carry lookahead value is computed at the left and used to compute bit 15. Note that the number of carry blocks decreases as the number of carry lookahead gates increases. For example, output 6 depends on three inc/dec blocks and no carry lookahead gates, while output 14 depends on one inc/dec block and two carry lookahead gates. If the inc/dec blocks and carry lookahead gates require approximately the same time, then the output bits will be ready at approximately the same time.

Schematic of the incrementer/decrementer circuit in the Z-80 microprocessor.

Schematic of the incrementer/decrementer circuit in the Z-80 microprocessor.

The image below shows what the incrementer/decrementer looks like physically, zooming in on the die photograph at the top of the article. The layout on the chip is slightly different from the schematic above. On the chip, the bits are arranged vertically with the low-order bit on top and the high-order bit on the bottom.

The image is a composite: the upper half is from the Z-80 die photograph, while the lower half shows the chip layers as tediously redrawn by the Visual 6502 team for analysis. You can see 8 horizontal "slices" of circuitry from top to bottom, since the bits are processed two at a time. The vertical metal wires are most visible (white in the photograph, blue in the layer drawing). These wires provide power, ground, control signals, and collect the lookahead carry from multiple bits. The polysilicon wires are reddish-orange in the layer diagram, while the diffused silicon is green. Transistors result where the two cross. If you look closely, you can see diagonal orange polysilicon wires about halfway across; these connect the carry-out from one bit to carry-in of the next.

The increment/decrement circuit in the Z-80 microprocessor. Top is the die photograph. Bottom is the layer drawing.

The increment/decrement circuit in the Z-80 microprocessor. Top is the die photograph. Bottom is the layer drawing.

Incrementing the refresh register

The refresh register R and interrupt vector I form a 16-bit pair. The refresh register gets incremented on every memory refresh cycle, but why doesn't the I register get incremented too? This would be a big problem since the value in the I register would get corrupted. The answer is the refresh input into the first carry lookahead gate in the schematic. During a refresh operation, a 1 value is fed into the gate here. This forces the carry to 0, stopping the increment at bit 6, leaving the I register unchanged (along with the top bit of the R register).

You might wonder why only 7 bits of the 8-bit refresh register get incremented. The explanation is that dynamic RAM chips store values in a square matrix. For refresh, only the row address needs to be updated, and all memory values in that row will be refreshed at once. When the Z-80 was introduced, 16K memory chips were popular. Since they held 2^14 bits, they had 7 row address bits and 7 column address bits. Thus, a 7 bit refresh value matched their need. Unfortunately, this rapidly became obsolete with the introduction of 64K memory chips that required 8 refresh bits. [Edit: it's a bit more complicated and depends on the specific chips. See the comments.] Some later chips based on the Z-80, such as the NSC800 had an 8-bit refresh to support these chips.

The non-increment feature

One unexpected feature of the Z-80's incrementer is that it can pass the value through unchanged. If the carry-in to the incrementer/decrementer is set to 0, no action will take place. This seems pointless, but it actually useful since it allows a 16-bit value to be latched and then read back unchanged. In effect, this provides a 16-bit temporary register. The Z-80 uses this action for EX (SP), HL, LD SP, HL, and the associated IX and IY versions. For the LD SP, HL, first HL is loaded into the incrementer latch. Then the unincremented value is stored in the SP register.

The EX (SP), HL is more complex, but uses the latch in a similar way. First the values at (SP+1) and (SP) are read into the WZ temporary register. Next the HL value is written to memory. Finally, WZ is loaded into the incrementer latch and then stored in HL.

You might wonder why values aren't copied between two registers directly. This is due to the structure of the register cells: they do not have separate load and store lines. Instead when a register is connected to the internal register bus, it will be overwritten if another value is on the bus, and otherwise it can be read. Even a simple register-to-register copy such as LD A,B cannot happen directly, but copy the data via the ALU. Since the Z-80's ALU is 4 bits wide, copying a 16-bit value would take at least 4 cycles and be slow. Thus, copying a 16-bit value via the incrementer latch is faster than using the WZ temporary registers.

One timing consequence of using the incrementer latch for 16-bit register-to-register transfers is that it cannot be overlapped with the instruction fetch. Many Z-80 instructions are pipelined and don't finish until several cycles into the next instruction, since register and ALU operations can take place while the Z-80 is fetching the next instruction from memory. However, the PC uses the incrementer during instruction fetch to advance to the next instruction. Thus, any transfer using the incrementer latch must finish before the next instruction starts.

The 0x0001 detector

Another unexpected feature of the incrementer/decrementer is it has a 16-input gate to test if the input is 0x0001 (not shown on the schematic). Why check for 1 and not zero? This circuit is used for the block transfer and search instructions mentioned earlier (LDIR, LDDR, CPIR, CPDR). These operations repeat a transfer or compare multiple times, decrementing the BC register until it reaches zero. But instead of checking for 0 after the decrement, the Z-80 checks if BC register is 1 before the decrement; this works out the same, but gives the Z-80 more time to detect the end of the loop and wrap up instruction execution.

No flags

Unlike the ALU, the incrementer/decrementer doesn't compute parity, negative, carry, or zero values. This is why the 16-bit increment/decrement instructions don't update the status flags.

Comparison with the 6502 and 8085

The 6502 has a 16-bit incrementer, but it is part of the program counter circuit. The 6502 only provides an incrementer, not a decrementer, as the PC doesn't need to be decremented. The other registers are 8 bits, so they don't need a 16-bit incrementer, but use the ALU to be incremented or decremented. (See the 6502 architecture diagram.) The 6502's incrementer uses a couple tricks for efficiency. It uses carry lookahead: the carry from the lowest 8 bits is computed in parallel, as is the carry from the next 4 bits. Alternating bits use a slightly different circuit to avoid inverters in the carry path, slightly reducing the propagation delay.

I've examined the 8085's register file and incrementer in detail. The incrementer/decrementer is implemented by a chain of half-adders with ripple carry. The 8085 has controls to select increment or decrement, similar to the Z-80. The 8085 also includes a feature to increment by two, which speeds up conditional jumps. As in the 6502, an optimization in the 8085 is that alternating bits are implemented with different circuits and the carry out of even bits is inverted. This avoids the inverters that would otherwise be needed to flip the carry back to its regular state. The 8085 uses the carry out from the incrementer to compute the undocumented K flag value.

Conclusion

Looking at the actual circuit for the incrementer/decrementer in the Z-80 shows the performance optimizations in a real chip, compared to a simple incrementer. The 6502 and 8085 also optimize this circuit, but in different ways. In addition, examining the circuitry sheds light on how some operations are implemented in the Z-80, as well as the way memory refresh was handled.

Credits: This couldn't have been done without the Visual 6502 team especially Pavel Zima, Chris Smith, Ed Spittles, Phil Mainwaring, and Julien Oster.