7 ms·
The Z-80 has a 4-bit ALU. Here's how it works.
- acegopher 13y agoThis is awesome. I'm going through the Elements of Computing Systems book/course (a.k.a. From NAND to Tetris) http://www.nand2tetris.org/ http://www.nand2tetris.org/ and it's been great in helping me understand how CPU's are constructed. The course actually has you make a ALU from logic gates, so you understand at a deep level just how it's done.
- kens 13y agoThat looks like a great course. I find it interesting how real processors mostly use the same principles you learn in school, but then they throw in clever tricks and optimizations that you never learn about. And every processor I've looked at (6502, 8085, Z-80) has its own style.
- pslam 13y agoCPUs of this era were normally multi-cycle for every instruction, but I never expected in the Z-80 at least one cycle was because the ALU was only 4 bit. Love the detailed analysis - and this is just the tip of the iceberg of that site. One thing I'm missing from this article is an approximate gate count. Obviously going 4 bit was motivated by gate and area saving, but halving the ALU size isn't going to halve the gate count or area, because it still needs the same width bus and extra latches for the partial answer. Or was it critical path? What kind of saving was it from an 8 bit ALU?
- kens 13y agoI don't have a gate count (yet). You're right that the 4-bit ALU doesn't save a lot of space overall. The Z-80 designer talks a bit about the 4-bit ALU [1] but doesn't really explain the motivation. My guess was he was able to use two cycles for the ALU without increasing the overall cycle count because memory cycles were the bottleneck. If you can cut the ALU in half "for free", why not? Hopefully as I continue analyzing the chip this will become clearer. [1] See page 10 in http://archive.computerhistory.org/resources/access/text/Oral_History/102658073.05.01.acc.pdf http://archive.computerhistory.org/resources/access/text/Ora... Note: if you're interested in Z-80 architecture, you seriously should read that link.
- ChuckMcM 13y agoOne of the reasons I was told was that the circuit extended to 16 bits easily (and was later used in the Z8000 as I recall) and doing decimal (BCD) math was easier. DAA (decimal adjust accumulator) was driven by the half carry flag. In '85 Intel wrote a Z80 emulator in 8086 machine code to try to win some Japanese game console design win and the decimal arithmetic stuff[1] was a PITA (and as it turned out not used a lot in games :-) [1] The 8080 also had these decimal arithmetic hacks but it didn't have an alternate set of registers to pull from.
- kens 13y agoThanks for the interesting information. I'm skeptical that the Z-80 designers were planning ahead for 16 bits, though. Simpler BCD math is a possibility - I'll look into this as I examine the Z-80 more. The 6502 wins, though, for crazy but efficient decimal arithmetic - it has a complex patented circuit that detects decimal carry in parallel with the addition/subtraction, and another circuit to add the correction factor to the result without going through the ALU again. So you don't need a separate DAA instruction or additional cycles for decimal correction. General question: what things about the Z-80 would you guys like me to write about? Any particular features of the chip? Register-level architecture, gates, or the silicon? Analyzing instructions do cycle by cycle? Gate counts by category? Comparison with other microprocessors?
- gp2000 13y agoWould love any and all analysis, but most interesting to me would be instruction details and especially the undocumented side effects. I'd also like to see comparison with the 8080 and how Zilog improved/changed the design.
- jloughry 13y ago...what things about the Z-80 would you guys like me to write about? Undocumented instructions! The MOS 6502 had plenty of these and I understand the Z-80 did too.
- jrabone 13y ago
- rwmj 13y agoIndeed .. in all the years I spent programming the Z80, I never for a minute suspected it had this ALU architecture. It's not even mentioned in Rodney Zaks's great work.
- rbanffy 13y agoI remember the Z80 felt distinctively more sluggish than the 6502 (I had an Apple II with a Z-80 Softcard in it so it could run CP/M). Now I know why.
- billforsternz 13y agoNo you don't, this was a clever optimization not a performance degrading hack. It was possible to save half the ALU transistors "for free" so the designer did. The for free bit is important. The Z80 ran a superset of the 8085 instruction set at equal or greater speed, but the 8085 had an 8 bit ALU.
- rbanffy 13y agoThe minimum time of an instruction to execute on the 6502 was 2 clock cycles. The maximum is 7, IIRC. On the Z80, it's 4, with the maximum being about 30. This has, of course, little to do with the width of the ALU.
- DonGateley 13y agoI think it started life as a 4 bit processor to compete with the 4004 and went out the door as 8 after the 8080 so they just muxed what was already there. I don't know that, I just think that. :-)
- rayiner 13y agoThe Pentium 4 also had a 16-bit ALU, which computed a 16-bit operation on each of the rising and falling edges of the clock to maintain 1-cycle latency. www.cs.virginia.edu/~mc2zk/cs451/mco_P4.ppt.
- stephengillie 13y agoThe requested URL /~mc2zk/cs451/mco_P4.ppt‎ was not found on this server. I don't know who is mangling the URL (Chrome, Apache, MITM?) nor why it's happening.
- ashmud 13y agoThere's a U+200E character between "www.cs.virginia.edu/~mc2zk/cs451/mco_P4.ppt" and "."
- kens 13y agoThere's an invisible Unicode U+200E left-to-right mark at the end of the URL, probably picked up when the parent cut-and-paste the URL into HN. This in UTF-8 is E2 80 8E, which gets misinterpreted by the server as Windows-1252 character set: E2 = â, 80 = €, 8e = Ž. (It could be iso-8859-1, except that doesn't include €.) Interestingly, Chrome's DOM inspector shows this character as the HTML entity ‎ while view-source has it as the actual invisible character. I think the poster of the URL originally mangled it, but it would be nice if the HN software filtered out invisible characters from URLs. There's not much the destination server can do about it. (Yes, I've dealt with too many character set issues in the past.)
- rayiner 13y agohttp://www.cs.virginia.edu/~mc2zk/cs451/mco_P4.ppt http://www.cs.virginia.edu/~mc2zk/cs451/mco_P4.ppt Does that work? I copied and pasted it from a Google search result (because otherwise the file downloads without showing me the URL). Google of course has decided "copy link to" shouldn't work.
- nitrogen 13y agoGoogle of course has decided "copy link to" shouldn't work. There's an addon for that.
- beachstartup 13y agoin high school i learned z80 assembly to hack games on my ti-86 calculator. it's a great chip to learn on. i remember there was a cross-compiler and a software utility + serial cable ... after some googling: http://www.ticalc.org/programming/columns/86-asm/el-helw/lesson1.html http://www.ticalc.org/programming/columns/86-asm/el-helw/les...
- jdmichal 13y agoThis is also very useful for anyone who's ever worked with a Gameboy emulator. It uses a processor very similar to the Z-80, including this particular ALU setup.