3 ms·

I was a 6502 guy back in the 80s, before I went 68k, and never had a machine with the Z80, so have no nostalgia for it. But when I look on paper, the Z80 really

I was a 6502 guy back in the 80s, before I went 68k, and never had a machine with the Z80, so have no nostalgia for it. But when I look on paper, the Z80 really does look like it stood the test of time better. How does the eZ80 extend out into 24 bits without having an awkward banking system like the '816? (EDIT: I see, it actually adds a 24-bit register mode.) The biggest problem I found with the 816 was that having no registers larger than 16-bits meant, well, there's no way to store a pointer in a register, or work with data larger than 64k. Well, that and the awful 8/16 mode switching. Oh, and the fact that the stack and direct page are stuck in the first 64k bank. Damn, I really wish Mensch had designed and built a 65xx with a proper 16bit (or higher) architecture instead of the meh that is the 816.

alexisread 3y ago
I'd imagine the ez80 uses an extended register fusion setup ie. You can use 3 8bit registers as a 24bit register. I think that would take 3 cycles to use the full address space, but for the first 8bit space I guess it would work like utf8 in that you just use 1 register (byte).
PaulHoule 3y ago
They extended the length of the registers for the eZ80, which they didn’t do for the 816, it is explained really well here http://www.zilog.com/docs/um0077.pdf http://www.zilog.com/docs/um0077.pdf I made it through (most of) the 1980s with a 6809-based TRS-80 Color Computer but I think the 6502 was the most popular CPU in the US then, as the 6502 based PET, Apple ][, C64, Atari 400, Atari 2600 and NES used them. I think it was rare to see Z-80 computers with color graphics, framebuffers, sprites, etc. but it was used in arcade boards and they had this https://en.wikipedia.org/wiki/MSX https://en.wikipedia.org/wiki/MSX in some other countries. I wonder if you could build a OS for eZ80 that could run CP/M software, that would get you a pretty large base of software to run. In an alternate timeline, Apple comes out with the iiGS a few years earlier than it did instead of the Mac but no the ‘816 was not that chip and they would have been looking for a replacement around the time they got looking for a 68k replacement.
- pkaye 3y ago
  I looked it up and the eZ80 is instruction set compatible with Z80 so I think it could run old CP/M binaries. You would have to customize the BIOS to add support any IO devices.
- cmrdporcupine 3y ago
  When I started to look at the instruction set of the 816 more closely (and a coworker of mine was writing an emulator at the same time) it became clearer to me why Mensch went the route he did; a) there just wasn't room in the 8-bit instruction encoding in the '02 to cleanly add 16-bit (or 24-bit etc) versions of each instruction. So instead of moving out into multibyte instructions, he introduced the mode switch and b) he seems to have done this by doing very minimal surgery to the 6502 core itself, basically adding some 16-bit shunts (depending on mode) in front of the decoder to just change the register sizes and behaviors really. (apart from the improvements like adding relocatable direct page and movable stack, like the 6809 got) c) apple wanted something that fit in a 40-pin DIP, to fit into existing motherboard design constraints presumably... and for it to support full 6502 compatibility. so we got this awful address bus multiplexing thing, and even after moving up into other packaging, he never fixed that (oddly he later made a microcontroller, the 65C265, that has all 24 pins without multiplexing). If he'd just started with PDIP-64 like the MC68000, so much agony would have been saved. I think it's a shame really. We could have had a 16/24 version of the 6502 with fewer compromises if the 816 had been a more "serious" project from Apple. But at the time the IIgs was in progress and WDC commissioned to work on the chip, I suspect there was already the politics within Apple about killing the Woz-line in favour of the 68k Mac. We had IIgs systems in my primary school, and they only ever got used as Apple II clones. I had an Atari ST, and while the 68000 was a great chip, it was clearly targeted towards "VAX" class or workstation type applications originally. Its overall complexity is quite high, and interrupt responsiveness very slow, when compared to the 6502. It would have been interesting to see a Mensch-designed 16/[24/32]-bit processor that kept the same minimalist philosophy as the 6502. Paired with a chipset like what was in the Amiga, I think it would been really cool. Though maybe the ARM line is that, in some ways.
ralphc 3y ago
Back in the day I bought a TRS-80 instead of an Apple ][ based on how much easier the Z-80 looked to write assembly for.

alexisread 3y ago

I'd imagine the ez80 uses an extended register fusion setup ie. You can use 3 8bit registers as a 24bit register. I think that would take 3 cycles to use the full address space, but for the first 8bit space I guess it would work like utf8 in that you just use 1 register (byte).

PaulHoule 3y ago

They extended the length of the registers for the eZ80, which they didn’t do for the 816, it is explained really well here http://www.zilog.com/docs/um0077.pdf http://www.zilog.com/docs/um0077.pdf I made it through (most of) the 1980s with a 6809-based TRS-80 Color Computer but I think the 6502 was the most popular CPU in the US then, as the 6502 based PET, Apple ][, C64, Atari 400, Atari 2600 and NES used them. I think it was rare to see Z-80 computers with color graphics, framebuffers, sprites, etc. but it was used in arcade boards and they had this https://en.wikipedia.org/wiki/MSX https://en.wikipedia.org/wiki/MSX in some other countries. I wonder if you could build a OS for eZ80 that could run CP/M software, that would get you a pretty large base of software to run. In an alternate timeline, Apple comes out with the iiGS a few years earlier than it did instead of the Mac but no the ‘816 was not that chip and they would have been looking for a replacement around the time they got looking for a 68k replacement.

pkaye 3y ago
I looked it up and the eZ80 is instruction set compatible with Z80 so I think it could run old CP/M binaries. You would have to customize the BIOS to add support any IO devices.
cmrdporcupine 3y ago
When I started to look at the instruction set of the 816 more closely (and a coworker of mine was writing an emulator at the same time) it became clearer to me why Mensch went the route he did; a) there just wasn't room in the 8-bit instruction encoding in the '02 to cleanly add 16-bit (or 24-bit etc) versions of each instruction. So instead of moving out into multibyte instructions, he introduced the mode switch and b) he seems to have done this by doing very minimal surgery to the 6502 core itself, basically adding some 16-bit shunts (depending on mode) in front of the decoder to just change the register sizes and behaviors really. (apart from the improvements like adding relocatable direct page and movable stack, like the 6809 got) c) apple wanted something that fit in a 40-pin DIP, to fit into existing motherboard design constraints presumably... and for it to support full 6502 compatibility. so we got this awful address bus multiplexing thing, and even after moving up into other packaging, he never fixed that (oddly he later made a microcontroller, the 65C265, that has all 24 pins without multiplexing). If he'd just started with PDIP-64 like the MC68000, so much agony would have been saved. I think it's a shame really. We could have had a 16/24 version of the 6502 with fewer compromises if the 816 had been a more "serious" project from Apple. But at the time the IIgs was in progress and WDC commissioned to work on the chip, I suspect there was already the politics within Apple about killing the Woz-line in favour of the 68k Mac. We had IIgs systems in my primary school, and they only ever got used as Apple II clones. I had an Atari ST, and while the 68000 was a great chip, it was clearly targeted towards "VAX" class or workstation type applications originally. Its overall complexity is quite high, and interrupt responsiveness very slow, when compared to the 6502. It would have been interesting to see a Mensch-designed 16/[24/32]-bit processor that kept the same minimalist philosophy as the 6502. Paired with a chipset like what was in the Amiga, I think it would been really cool. Though maybe the ARM line is that, in some ways.

ralphc 3y ago

Back in the day I bought a TRS-80 instead of an Apple ][ based on how much easier the Z-80 looked to write assembly for.