5 ms·
CPUs 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 t
by pslam 13y ago
CPUs 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 agoWhether to provide BCD optimisation always seemed to be a tricky engineering decision; virtually nobody used the 6502 BCD instructions in the amateur home microcomputer environment I was familiar with in the 80s, but it was clearly considered to be important to the CPU manufacturers. Were there BCD benchmarks back then? Was it considered a killer feature to make financial software easier to write? Did Rockwell ever capitalise on that patent?
- to3m 13y agoI have no idea what people used BCD for either. I vaguely recall reading that the C64's interrupt routine didn't even bother to clear the D flag, so you had to disable interrupts while using decimal mode! - so obviously most people just weren't expected to be using it. I only ever saw it used for game scores... and the following, which prints a byte as hex, and is a neat example of cute 6502 code. Saves a few bytes over having a table of hex digits, and you don't need to save X or Y. HEX: PHA LSR:LSR:LSR:LSR JSR HEX2 PLA AND #15 HEX2: CLC SED:ADC #$90:ADC #$40:CLD JMP PUTCH (PUTCH takes an ASCII character in A.) The 68000 had BCD as well. Never used it and don't recall ever seeing it used. I think they only included it so they could have an instruction called ABCD.
- mpyne 13y agoI would imagine BCD was useful as a bootstrap for a poor ASM programmer's bignum library (especially when 'bignum' was >16 bits). Also would be useful for 7-segment LED displays.
- pwg 13y agoThe Atari's ROM's contained a full (well, for the time) floating point library implementation that used BCD floating point values. The result was that the Atari's, without even trying, had more accurate decimal math algorithms than other contemporary computers. Something to do on the demo machines of the day in stores was to run this loop: 10 let x = 100 20 print x 30 let x = x - 0.01 40 goto 20 On an Atari this would accurately count down from 100 to zero with zero round off errors. The exact same loop on an IBM PC after about 5 steps started printing things like 99.94999999998 instead of 99.95. Edit: formatting
- 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. :-)