9 ms·

Z80, the 8-bit Number Cruncher (2011)

65 points by elvis70 6y ago

tyingq 6y ago
You can still buy new TI calculators powered by a Z80. Pretty long-lived platform.
- NanoWar 6y ago
  They are pretty amazing, because you can start with Ti-Basic and once you're hooked you will want to know all about writing assembly. Doing that with the rich ecosystem/libraries and community available is much easier than doing the same with just the processor alone. You can load your games (presumably) with USB and have cursor keys right below the screen yay!
  - tyingq 6y ago
    Unfortunately, TI did make ASM much harder to do earlier this year. There is a jailbreak for some OS versions: https://linustechtips.com/main/topic/1198899-texas-instruments-bans-all-asm-programs-games-on-ti-84-plus-ce-calculators/?tab=comments#comment-13635333 https://linustechtips.com/main/topic/1198899-texas-instrumen...
speeder 6y ago
People make low bit general purpose CPUs with modern processes? Or devices that use such CPUs still use ones with ancient designs?
- kevin_thibedeau 6y ago
  Most moderate complexity ICs have a microcontroller inside. Depending on performance needs an 8-bitter is sufficient. These will typically be a clone of an established architecture so that tooling is readily available.
  - speeder 6y ago
    Well... exactly. Thing is, since I am a game-dev: it would be cool to have a cheap console or portable console that runs at quite high speeds. For example what would be possible if I had a Z80 that ran at 2ghz?
    - rusk 6y ago
      > what would be possible ... Toasted Z80
    - buescher 6y ago
      The TI 84 Plus CE graphing calculator has a 48MHz eZ80, a 320x240 16-bit color display, 154KB of RAM, 3MB of flash, and a built-in rechargeable battery. No audio capabilities, though. Z80 machines of the 8-bit era were typically 2MHz or 4MHz.
    - PopePompus 6y ago
      Running an old 8-bit design at a very high clock speed would probably consume more power than a slower clocked ARM chip which could process as fast or faster due to the higher complexity of the chip. Using an old 8-bit design only makes sense when the computing requirements are extremely modest.
- hvs 6y ago
  They're cheap and easy to use. https://www.digikey.com/catalog/en/partgroup/z80/15507 https://www.digikey.com/catalog/en/partgroup/z80/15507
nickcw 6y ago
Nice article! Back in the day I wrote 10s of thousands of lines of Z80 assembler - it was my first actual paid job after university. You got to know each register - they have their own character - need a loop - well you need B and DJNZ, need programmable IO, well you need C then, etc I still have a soft spot for the Z80! Reading that article I learnt some great techniques! The code I was writing always used the alternate register bank for interrupt handling so you could have really fast save and restore registers. I never thought of using them to pass parameters and doing fast arithmetic - very neat! ; ADD ROUTINE 32+32BIT=32BIT ; H'L'HL = H'L'HL + D'E'DE ; CHANGES FLAGS ; ADD32: ADD HL,DE ; 16-BIT ADD OF HL AND DE EXX ADC HL,DE ; 16-BIT ADD OF HL AND DE WITH CARRY EXX RET
jhallenworld 6y ago
8088 was only three years later than z80 and it was much better than it. But cost also matters.. I just looked at 1982 Byte Magazine at the "JDR Microdevices" ad in the back: Z80A $6.00 Z80B $18.95 6502 $6.95 6502A $12.95 6809 $24.95 8088 $39.95
- Someone 6y ago
  “Only three years” is two doublings according to Moore’s law, and that law held fairly strongly at the time (transistor counts, according to https://en.wikipedia.org/wiki/Transistor_count https://en.wikipedia.org/wiki/Transistor_count: 8,500 (“6,813 without depletion mode pull-up transistors”) vs 29,500), so of course it was much better.
  - klelatti 6y ago
    For me the it's disappointing how small a step forward the 8088 was from the Z80 given the increase in the number of transistors. It came a few years later but the ARM1 which used 25,000 transistors and was built on roughly the same process size [1] and was much faster than the 8086/8088 shows what might have been had Intel gone down a RISC route. [1] https://en.wikichip.org/wiki/acorn/microarchitectures/arm1 https://en.wikichip.org/wiki/acorn/microarchitectures/arm1
    - jhallenworld 6y ago
      I think you needed cheaper RAM also to enable RISC. At the very least, a 32-bit wide interface was not going to fly this early.
      - klelatti 6y ago
        You're absolutely right: ARM1 code density would have been much, much worse than 8088 (which Thumb partly solved later on) and the 32 bit bus wouldn't have been practical. Edit: By complete coincidence browsing some related Z80 came across your memcpy comparison which is fascinating. The step up to the ARM1 is remarkable. I think I remember Herman Hauser saying that Acorn approached Intel for an x86 chip with higher memory bandwidth but were rebuffed and that led to the development of Arm.
ronald_petty 6y ago
I just found a book at an electronics store yesterday for the Z80. I believe it was copyright 1980. Nice post.
mark-r 6y ago
One trick I used a lot with the Z80 is the conditional return instructions. Instead of doing a jump, structure your functions so you could return early on one of the condition flags. The jump instructions were 3 bytes, the return instructions were only 1.
- sedatk 6y ago
  It was only useful when stack wasn’t involved I presume?
  - rusk 6y ago
    Not sure about Z80 but stack is a bit of a luxury on 6502 ...
    - gp2000 6y ago
      The Z-80 stack pointer (SP) is a full 16 bit register. The stack can be anywhere in memory and be as large as needed. The 6502 has only an 8 bit stack pointer which points into page 1 only (addresses 0x100 .. 0x1FF).
    - mark-r 6y ago
      The Z80 stack was only limited by your memory size.
- gp2000 6y ago
  And it could also save time. The Z-80 conditional return takes 5 cycles (or "T-States" in Zilog terminology) to execute if the condition is not met and 11 cycles otherwise. Quite worthwhile in testing for uncommon cases. Consider this routine to print a message that is terminated by nul or newline: PMESS LD A,(HL) ; get character from message INC HL ; move message pointer to next character CP 10 ; newline? RET Z ; return if so OR A ; zero? RET Z ; done if so CALL PCHAR ; print character JP PMESS ; keep looping If instead of "RET Z" we had to do a conditional jump to a return it would be 10 cycles for each test instead of 5. CP 10 JP Z,DONE ; 10 cycles, jump taken or not ... JP PMESS DONE: RET ; 10 cycles, BTW The conditional return just happens to be cheaper if not taken because it skips the work of popping the return address off the stack. Though purely an outcome of the implementation you can treat it as sort of a branch prediction. Incidentally, the Z-80 also has relative branches (JR) that differ in execution time whether they are taken or not. The branch offset is a single byte so JR is only 2 bytes compared to JP's 3. A JR is 7 cycles if not taken, 12 otherwise. Again, we can treat it as a hard-coded branch prediction that predicts the branch is not taken. If space or distance to target is not a problem, a JR is faster if taken less than 60% of the time.
astatine 6y ago
I had built (with a couple of other people) a TCP/IP stack for the Z80 in the late 90s. And a small HTTP server to go with it. It had a weird CGI like interface to call some subroutine for a path. The intention was to use it in some kind of industrial equipment where users could directly point their browser to this HTTP server and get status updates. A super simple IOT, if you will. This was actually for Zilog itself who were positioning the (already faded) chip for embedded networking. It was perfect for such applications! Good memories.
- jhallenworld 6y ago
  Dataprobe's original "iBoot" worked like this: http://dataprobe.com/iboot-hub/ http://dataprobe.com/iboot-hub/ It had a built-in web server running on an 8051! It interfaced with an Ethernet chip using a parallel FIFO interface- maybe NE2000 based chip? I don't remember..
  - astatine 6y ago
    I had a similar parallel interface to an Ethernet daughterboard. I too don't recollect the make. But there was a serial PPP based option as well - it made perfect sense when main stream modems were anyway 9600 baud or lesser.
hsitz 6y ago
The Z80 was the chip in the original Radio Shack TRS-80 computers. I got one back in 1979, when I was 15 years old. I had a book on the Z80, the text in this article brings back memories, and figured out how to do some simple machine language programming. Mostly simple graphics programs, storing values to the TRS-80's display memory buffer. Eventually I got the TRS-80 Assembler that made things a bit easier. I can't remember now how I actually input the machine language hexadecimal without using that assembler, but I'm sure I did. Or maybe not, my memory is sometimes not so reliable on these things, even when I feel certain. I can hardly imagine how different things must be now for young kids growing up in internet era, where everything is at your fingertips. Information was much more difficult to get back in the old days, traveled much more slowly. But, still, kids had the energy to figure things out.
- java-man 6y ago
  Now, imagine the thrill of holding an original Intel 8080 data book behind the Iron Curtain. With impeccably drawn diagrams, on high quality paper, in stark contrast with anything produced by the Soviet industry.
klelatti 6y ago
I loved writing Z80 assembly back in the 1980s but it was a bit of as shock to find out from Ken Shirriff [1] that this 'number cruncher' had a 4 bit ALU. [1] http://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-how-it.html http://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-h... EDIT: Just to be clear this is in no way intended to be a criticism of the Z80. On the contrary it's a tribute to the ingenuity of Federico Faggin and Masatoshi Shima that they were looking for ways to 'save' transistors in the ALU in a way which no-one would notice, which presumably enabled them to add more features elsewhere.
- ncmncm 6y ago
  The Z80 is not above criticism. Its fancy loop instructions were slower than a regular loop. But the 4-bit ALU was the result of a very astute observation that inefficiencies in the 8080 microachitecture would hide an extra ALU cycle.
Snitch-Thursday 6y ago
Okay guys, this may be off-topic, not sure as I've not finished the article yet. What (wiki?)books can I purchase that would help me bridge my mental understanding of computer circuitry from 'this is a circuit with a resistor and a power source and a lightbulb, while this is a CPU that has wires to memory and magic happens to make em talk' to 'I know the difference between TTL logic and a microprocessor and can fuzzily trace where the electrical signals go'? I don't own but am aware of things called logic analyzers, 'progammers', and the difference between a microcontroler and a microprocessor (ish, the former is vaguely a system on a chip?), but I'm not sure where to start to fill in the many gaps in the bits of knowledge I have so far. Wikipedia feeds me articles about logic gates, programmable logic, and Algorithmic Logical Units used in things like the Xerox Alto, but I don't know if this means I should try for electrical engineering courses or something simpler?
- ggambetta 6y ago
  I don't know about books, but I learned about this in my first year in university (for CS-ish), and it was fascinating :) I think the missing link you're looking for is the flip-flop [0]. It's a clever arrangement of logic gates (just AND, OR, NOT, etc) that can store a bit. The concept of a clock signal is very closely related. Then you learn about multiplexers and you realize that if you put a bunch of flip-flops that can store a bit behind a multiplexer [1], suddenly you have something that looks like very primitive RAM with a data and address bus! At the time it blew my mind that you can (conceptually, at least) take a bunch of logic gates, arrange them in clever ways, and arrive pretty quickly at much "higher level" concepts like memory on an address bus. [0] https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flop https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip... [1] https://en.wikipedia.org/wiki/Multiplexer#Digital_multiplexers https://en.wikipedia.org/wiki/Multiplexer#Digital_multiplexe...
- mrspeaker 6y ago
  I'm not sure what you're looking for (maybe I mis-read your requirements above), but for the step from "I understand the idea of simple boolean logic" to "I understand (quite concretely) how a Z80 (or 6502 or similar) CPU works", I'd recommend the book "The Elements of Computing Systems" from the Nand2Tetris site: https://www.nand2tetris.org/book https://www.nand2tetris.org/book It's what made things click for me. [EDIT: I also second Klelatti's suggestion... Ben Eater's youtube channel is ALL gold. He's an amazing teacher. I also purchased his clock module, and the 6502 kit - they are awesome!]
klelatti 6y ago
For anyone interested in Z80 assembly, the ZX Spectrum ROM disassembly, recently discussed on HN [1] is a treasure trove of annotated Z80 code including a full set of floating point routines. [1] https://news.ycombinator.com/item?id=23757449 https://news.ycombinator.com/item?id=23757449
ralphc 6y ago
1977 me had to choose between a TRS-80 and an Apple ][. Apple had the games & color but man, the Z80 was so much nicer for assembly programming than the 6502. Z80 assembly and TRS-80 BASIC got me started on a lifelong career and passion.
s_gourichon 6y ago
Interesting article. Indeed the Z80 was used in the Amstrad CPC series. There, the firmware used the alternate register set for bookkeeping, so you could use them only in a code region surrounded with interrupt dis/enabling and saving/restoring them. The floating point format on the CPC is 40 bits, probably one of the formats mentioned in the article. A few people still create prod for the Z80 every year, there are already 10 productions registered in 2020 on http://www.pouet.net/prodlist.php?platform%5B%5D=Amstrad+CPC&page=1 http://www.pouet.net/prodlist.php?platform%5B%5D=Amstrad+CPC... The CPC had a huge graphics memory for the time: 16k, making nice-looking games in paper catalog but very limited full-screen animation without piles of hacks. Lots of actual number crunching for 3D graphics in thi old prod "All in 3D" https://www.pouet.net/prod.php?which=14667 https://www.pouet.net/prod.php?which=14667 (no video, you have to run it e.g. in an emulatoe). For an exceptional result of Z80 performing number crunching and graphical hacks, see this nice demo from 2017 "Amstrad CPC demo : Logon's run - 3D meets the aging bits" https://www.youtube.com/watch?v=22wSm4y27Wk https://www.youtube.com/watch?v=22wSm4y27Wk . Or "Batman Forever" demo of 2011 https://www.youtube.com/watch?v=_syHewDu5lc https://www.youtube.com/watch?v=_syHewDu5lc I personally made a production this year https://gourichon.org/cpcitor/justget9/beta/ https://gourichon.org/cpcitor/justget9/beta/ a puzzle game with smooth animation, which got awarded at a local contest https://www.facebook.com/groups/1120607071477318/permalink/1526248760913145/ https://www.facebook.com/groups/1120607071477318/permalink/1...
elvis70 6y ago
I noticed an error in the "Z80 IF-ELSE" section. It says that the overflow flag is the equivalent of the carry flag for signed numbers. But this is wrong. The overflow flag indicates that the result of the signed operation is wrong. In fact, it is the sign and zero flags that should be tested. In case of overflow, the sign must be reversed. See this document about the carry and overflow flags: http://teaching.idallen.com/dat2343/11w/notes/040_overflow.txt http://teaching.idallen.com/dat2343/11w/notes/040_overflow.t... The code for comparing HL >= DE should be: (Replaced JP PO by JP M + inversion of the sign if overflow) ;================================================== ; IF-ELSE CODE FRAGMENT ; LD HL,-2000 ; HL = -2000; LD DE,-1000 ; DE = -1000; AND A ; if (HL >= DE) { SBC HL,DE JP PO,NOOV LD A,H ; INVERSE SIGN BIT IF OVERFLOW XOR 80H ; NOOV: JP M,ELSE LD A,1 ; A = 1; JR ENDIF ELSE: ; } else { LD A,0 ; A = 0; ENDIF: ; } HALT END

65 points by elvis70 6y ago

tyingq 6y ago

You can still buy new TI calculators powered by a Z80. Pretty long-lived platform.

NanoWar 6y ago
They are pretty amazing, because you can start with Ti-Basic and once you're hooked you will want to know all about writing assembly. Doing that with the rich ecosystem/libraries and community available is much easier than doing the same with just the processor alone. You can load your games (presumably) with USB and have cursor keys right below the screen yay!
- tyingq 6y ago
  Unfortunately, TI did make ASM much harder to do earlier this year. There is a jailbreak for some OS versions: https://linustechtips.com/main/topic/1198899-texas-instruments-bans-all-asm-programs-games-on-ti-84-plus-ce-calculators/?tab=comments#comment-13635333 https://linustechtips.com/main/topic/1198899-texas-instrumen...

speeder 6y ago

People make low bit general purpose CPUs with modern processes? Or devices that use such CPUs still use ones with ancient designs?

kevin_thibedeau 6y ago
Most moderate complexity ICs have a microcontroller inside. Depending on performance needs an 8-bitter is sufficient. These will typically be a clone of an established architecture so that tooling is readily available.
- speeder 6y ago
  Well... exactly. Thing is, since I am a game-dev: it would be cool to have a cheap console or portable console that runs at quite high speeds. For example what would be possible if I had a Z80 that ran at 2ghz?
  - rusk 6y ago
    > what would be possible ... Toasted Z80
  - buescher 6y ago
    The TI 84 Plus CE graphing calculator has a 48MHz eZ80, a 320x240 16-bit color display, 154KB of RAM, 3MB of flash, and a built-in rechargeable battery. No audio capabilities, though. Z80 machines of the 8-bit era were typically 2MHz or 4MHz.
  - PopePompus 6y ago
    Running an old 8-bit design at a very high clock speed would probably consume more power than a slower clocked ARM chip which could process as fast or faster due to the higher complexity of the chip. Using an old 8-bit design only makes sense when the computing requirements are extremely modest.
hvs 6y ago
They're cheap and easy to use. https://www.digikey.com/catalog/en/partgroup/z80/15507 https://www.digikey.com/catalog/en/partgroup/z80/15507

nickcw 6y ago

Nice article! Back in the day I wrote 10s of thousands of lines of Z80 assembler - it was my first actual paid job after university. You got to know each register - they have their own character - need a loop - well you need B and DJNZ, need programmable IO, well you need C then, etc I still have a soft spot for the Z80! Reading that article I learnt some great techniques! The code I was writing always used the alternate register bank for interrupt handling so you could have really fast save and restore registers. I never thought of using them to pass parameters and doing fast arithmetic - very neat! ; ADD ROUTINE 32+32BIT=32BIT ; H'L'HL = H'L'HL + D'E'DE ; CHANGES FLAGS ; ADD32: ADD HL,DE ; 16-BIT ADD OF HL AND DE EXX ADC HL,DE ; 16-BIT ADD OF HL AND DE WITH CARRY EXX RET

jhallenworld 6y ago

8088 was only three years later than z80 and it was much better than it. But cost also matters.. I just looked at 1982 Byte Magazine at the "JDR Microdevices" ad in the back: Z80A $6.00 Z80B $18.95 6502 $6.95 6502A $12.95 6809 $24.95 8088 $39.95

Someone 6y ago
“Only three years” is two doublings according to Moore’s law, and that law held fairly strongly at the time (transistor counts, according to https://en.wikipedia.org/wiki/Transistor_count https://en.wikipedia.org/wiki/Transistor_count: 8,500 (“6,813 without depletion mode pull-up transistors”) vs 29,500), so of course it was much better.
- klelatti 6y ago
  For me the it's disappointing how small a step forward the 8088 was from the Z80 given the increase in the number of transistors. It came a few years later but the ARM1 which used 25,000 transistors and was built on roughly the same process size [1] and was much faster than the 8086/8088 shows what might have been had Intel gone down a RISC route. [1] https://en.wikichip.org/wiki/acorn/microarchitectures/arm1 https://en.wikichip.org/wiki/acorn/microarchitectures/arm1
  - jhallenworld 6y ago
    I think you needed cheaper RAM also to enable RISC. At the very least, a 32-bit wide interface was not going to fly this early.
    - klelatti 6y ago
      You're absolutely right: ARM1 code density would have been much, much worse than 8088 (which Thumb partly solved later on) and the 32 bit bus wouldn't have been practical. Edit: By complete coincidence browsing some related Z80 came across your memcpy comparison which is fascinating. The step up to the ARM1 is remarkable. I think I remember Herman Hauser saying that Acorn approached Intel for an x86 chip with higher memory bandwidth but were rebuffed and that led to the development of Arm.

ronald_petty 6y ago

mark-r 6y ago

One trick I used a lot with the Z80 is the conditional return instructions. Instead of doing a jump, structure your functions so you could return early on one of the condition flags. The jump instructions were 3 bytes, the return instructions were only 1.

sedatk 6y ago
It was only useful when stack wasn’t involved I presume?
- rusk 6y ago
  Not sure about Z80 but stack is a bit of a luxury on 6502 ...
  - gp2000 6y ago
    The Z-80 stack pointer (SP) is a full 16 bit register. The stack can be anywhere in memory and be as large as needed. The 6502 has only an 8 bit stack pointer which points into page 1 only (addresses 0x100 .. 0x1FF).
  - mark-r 6y ago
    The Z80 stack was only limited by your memory size.
gp2000 6y ago
And it could also save time. The Z-80 conditional return takes 5 cycles (or "T-States" in Zilog terminology) to execute if the condition is not met and 11 cycles otherwise. Quite worthwhile in testing for uncommon cases. Consider this routine to print a message that is terminated by nul or newline: PMESS LD A,(HL) ; get character from message INC HL ; move message pointer to next character CP 10 ; newline? RET Z ; return if so OR A ; zero? RET Z ; done if so CALL PCHAR ; print character JP PMESS ; keep looping If instead of "RET Z" we had to do a conditional jump to a return it would be 10 cycles for each test instead of 5. CP 10 JP Z,DONE ; 10 cycles, jump taken or not ... JP PMESS DONE: RET ; 10 cycles, BTW The conditional return just happens to be cheaper if not taken because it skips the work of popping the return address off the stack. Though purely an outcome of the implementation you can treat it as sort of a branch prediction. Incidentally, the Z-80 also has relative branches (JR) that differ in execution time whether they are taken or not. The branch offset is a single byte so JR is only 2 bytes compared to JP's 3. A JR is 7 cycles if not taken, 12 otherwise. Again, we can treat it as a hard-coded branch prediction that predicts the branch is not taken. If space or distance to target is not a problem, a JR is faster if taken less than 60% of the time.

astatine 6y ago

I had built (with a couple of other people) a TCP/IP stack for the Z80 in the late 90s. And a small HTTP server to go with it. It had a weird CGI like interface to call some subroutine for a path. The intention was to use it in some kind of industrial equipment where users could directly point their browser to this HTTP server and get status updates. A super simple IOT, if you will. This was actually for Zilog itself who were positioning the (already faded) chip for embedded networking. It was perfect for such applications! Good memories.

jhallenworld 6y ago
Dataprobe's original "iBoot" worked like this: http://dataprobe.com/iboot-hub/ http://dataprobe.com/iboot-hub/ It had a built-in web server running on an 8051! It interfaced with an Ethernet chip using a parallel FIFO interface- maybe NE2000 based chip? I don't remember..
- astatine 6y ago
  I had a similar parallel interface to an Ethernet daughterboard. I too don't recollect the make. But there was a serial PPP based option as well - it made perfect sense when main stream modems were anyway 9600 baud or lesser.

hsitz 6y ago

The Z80 was the chip in the original Radio Shack TRS-80 computers. I got one back in 1979, when I was 15 years old. I had a book on the Z80, the text in this article brings back memories, and figured out how to do some simple machine language programming. Mostly simple graphics programs, storing values to the TRS-80's display memory buffer. Eventually I got the TRS-80 Assembler that made things a bit easier. I can't remember now how I actually input the machine language hexadecimal without using that assembler, but I'm sure I did. Or maybe not, my memory is sometimes not so reliable on these things, even when I feel certain. I can hardly imagine how different things must be now for young kids growing up in internet era, where everything is at your fingertips. Information was much more difficult to get back in the old days, traveled much more slowly. But, still, kids had the energy to figure things out.

java-man 6y ago
Now, imagine the thrill of holding an original Intel 8080 data book behind the Iron Curtain. With impeccably drawn diagrams, on high quality paper, in stark contrast with anything produced by the Soviet industry.

klelatti 6y ago

I loved writing Z80 assembly back in the 1980s but it was a bit of as shock to find out from Ken Shirriff [1] that this 'number cruncher' had a 4 bit ALU. [1] http://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-how-it.html http://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-h... EDIT: Just to be clear this is in no way intended to be a criticism of the Z80. On the contrary it's a tribute to the ingenuity of Federico Faggin and Masatoshi Shima that they were looking for ways to 'save' transistors in the ALU in a way which no-one would notice, which presumably enabled them to add more features elsewhere.

ncmncm 6y ago
The Z80 is not above criticism. Its fancy loop instructions were slower than a regular loop. But the 4-bit ALU was the result of a very astute observation that inefficiencies in the 8080 microachitecture would hide an extra ALU cycle.

Snitch-Thursday 6y ago

Okay guys, this may be off-topic, not sure as I've not finished the article yet. What (wiki?)books can I purchase that would help me bridge my mental understanding of computer circuitry from 'this is a circuit with a resistor and a power source and a lightbulb, while this is a CPU that has wires to memory and magic happens to make em talk' to 'I know the difference between TTL logic and a microprocessor and can fuzzily trace where the electrical signals go'? I don't own but am aware of things called logic analyzers, 'progammers', and the difference between a microcontroler and a microprocessor (ish, the former is vaguely a system on a chip?), but I'm not sure where to start to fill in the many gaps in the bits of knowledge I have so far. Wikipedia feeds me articles about logic gates, programmable logic, and Algorithmic Logical Units used in things like the Xerox Alto, but I don't know if this means I should try for electrical engineering courses or something simpler?

ggambetta 6y ago
I don't know about books, but I learned about this in my first year in university (for CS-ish), and it was fascinating :) I think the missing link you're looking for is the flip-flop [0]. It's a clever arrangement of logic gates (just AND, OR, NOT, etc) that can store a bit. The concept of a clock signal is very closely related. Then you learn about multiplexers and you realize that if you put a bunch of flip-flops that can store a bit behind a multiplexer [1], suddenly you have something that looks like very primitive RAM with a data and address bus! At the time it blew my mind that you can (conceptually, at least) take a bunch of logic gates, arrange them in clever ways, and arrive pretty quickly at much "higher level" concepts like memory on an address bus. [0] https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flop https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip... [1] https://en.wikipedia.org/wiki/Multiplexer#Digital_multiplexers https://en.wikipedia.org/wiki/Multiplexer#Digital_multiplexe...
mrspeaker 6y ago
I'm not sure what you're looking for (maybe I mis-read your requirements above), but for the step from "I understand the idea of simple boolean logic" to "I understand (quite concretely) how a Z80 (or 6502 or similar) CPU works", I'd recommend the book "The Elements of Computing Systems" from the Nand2Tetris site: https://www.nand2tetris.org/book https://www.nand2tetris.org/book It's what made things click for me. [EDIT: I also second Klelatti's suggestion... Ben Eater's youtube channel is ALL gold. He's an amazing teacher. I also purchased his clock module, and the 6502 kit - they are awesome!]

klelatti 6y ago

For anyone interested in Z80 assembly, the ZX Spectrum ROM disassembly, recently discussed on HN [1] is a treasure trove of annotated Z80 code including a full set of floating point routines. [1] https://news.ycombinator.com/item?id=23757449 https://news.ycombinator.com/item?id=23757449

ralphc 6y ago

1977 me had to choose between a TRS-80 and an Apple ][. Apple had the games & color but man, the Z80 was so much nicer for assembly programming than the 6502. Z80 assembly and TRS-80 BASIC got me started on a lifelong career and passion.

s_gourichon 6y ago

Interesting article. Indeed the Z80 was used in the Amstrad CPC series. There, the firmware used the alternate register set for bookkeeping, so you could use them only in a code region surrounded with interrupt dis/enabling and saving/restoring them. The floating point format on the CPC is 40 bits, probably one of the formats mentioned in the article. A few people still create prod for the Z80 every year, there are already 10 productions registered in 2020 on http://www.pouet.net/prodlist.php?platform%5B%5D=Amstrad+CPC&page=1 http://www.pouet.net/prodlist.php?platform%5B%5D=Amstrad+CPC... The CPC had a huge graphics memory for the time: 16k, making nice-looking games in paper catalog but very limited full-screen animation without piles of hacks. Lots of actual number crunching for 3D graphics in thi old prod "All in 3D" https://www.pouet.net/prod.php?which=14667 https://www.pouet.net/prod.php?which=14667 (no video, you have to run it e.g. in an emulatoe). For an exceptional result of Z80 performing number crunching and graphical hacks, see this nice demo from 2017 "Amstrad CPC demo : Logon's run - 3D meets the aging bits" https://www.youtube.com/watch?v=22wSm4y27Wk https://www.youtube.com/watch?v=22wSm4y27Wk . Or "Batman Forever" demo of 2011 https://www.youtube.com/watch?v=_syHewDu5lc https://www.youtube.com/watch?v=_syHewDu5lc I personally made a production this year https://gourichon.org/cpcitor/justget9/beta/ https://gourichon.org/cpcitor/justget9/beta/ a puzzle game with smooth animation, which got awarded at a local contest https://www.facebook.com/groups/1120607071477318/permalink/1526248760913145/ https://www.facebook.com/groups/1120607071477318/permalink/1...

elvis70 6y ago

I noticed an error in the "Z80 IF-ELSE" section. It says that the overflow flag is the equivalent of the carry flag for signed numbers. But this is wrong. The overflow flag indicates that the result of the signed operation is wrong. In fact, it is the sign and zero flags that should be tested. In case of overflow, the sign must be reversed. See this document about the carry and overflow flags: http://teaching.idallen.com/dat2343/11w/notes/040_overflow.txt http://teaching.idallen.com/dat2343/11w/notes/040_overflow.t... The code for comparing HL >= DE should be: (Replaced JP PO by JP M + inversion of the sign if overflow) ;================================================== ; IF-ELSE CODE FRAGMENT ; LD HL,-2000 ; HL = -2000; LD DE,-1000 ; DE = -1000; AND A ; if (HL >= DE) { SBC HL,DE JP PO,NOOV LD A,H ; INVERSE SIGN BIT IF OVERFLOW XOR 80H ; NOOV: JP M,ELSE LD A,1 ; A = 1; JR ENDIF ELSE: ; } else { LD A,0 ; A = 0; ENDIF: ; } HALT END