Z80 REPL (2018)

Z80 REPL (2018)abagames.github.io

161 points•adunk•8 days ago•25 comments•

25 comments

The one thing I lamented about the Apple II Plus (vs. the original with Integer BASIC) was that to make room for autostart, they had to rip out the mini-assembler in the Monitor ROM. That was the closest thing I ever saw to an assembly-language REPL before this. At least they were able to keep the disassembler.

boomlinde8 days ago

It's common in the C64 world to use freezer cartridges that allow you to interrupt normal operation and invoke a monitor program. These typically include rudimentary assemblers and disassemblers as well. Fun and useful for patching and reverse engineering!

boomlinde8 days ago

Btw, I've only had a rough idea of how these are actually implemented, non-maskable interrupt something-something, and never knew how they make sure that the interrupt vector points to some persistent routine that can invoke the monitor. I found this great article explaining at least how The Final Cartridge 3 does it (in the first two sections): https://www.pagetable.com/282

Zeetah7 days ago

The Atari 8-bit computers could have a similar capability by using a product called Omnimon. Great way to look inside programs.

EvanAnderson8 days ago

I remember being so confused, as a 10 y/o kid, trying to follow examples from an Apple II-era book on my public library's Apple IIe computers. I didn't understand why I couldn't get the mini assembler to work. The idea that features would be removed on future machines simply didn't occur to me. I guess I really didn't even understand that the IIe was a different machine than what the book was written for since most everything else worked as-expected.

kevin_thibedeau8 days ago

The mini-assembler was still available from Integer BASIC on II+ and IIe. It returned to ROM for later IIe models.

MomsAVoxell6 days ago

Yes, I remember when this happened, it was a bummer .. but taught us the valuable lesson that the hardware manufacturer can give, but also take away. At least it kind of kicked off a tooling market ..

Incidentally, apropos onboard assemblies, in my retro collection I have a Commodore PC running an ancient DOS, and it is unique because of two reasons: it has DEBUG.com, and .. the machine is infected with one of the very early viruses of the era.

So, if you know the right path to take on boot, you can get the virus to infect things, under the debugger, and see it in action.

DEBUG.com was for me, a complete boon of a tool, so to speak, back in the day. It was just enough assembler/debugger to be able to do very effective things.

Anyway, Z80 REPL is delightful, I want a 6502 REPL now, and can't stop thinking about how to get one on the Atmos..

ErroneousBosh7 days ago

The BBC Micro and the Atom that came before it had an assembler built into BASIC. Just think, in the early 80s you could do inline assembly if your BASIC code wasn't fast enough...

MomsAVoxell6 days ago

13 year old me had a box of cassette tapes with index labels like "0 DEBUG", and "10 ASS", and "28 MEMHAK", "30 UNDEL", and so on, where the #'s were the position - on tape - of the little bundle of utilities I would auto-load as soon as I got home from school, while having dinner, etc. I spent days, dreaming of having a ROM burner to put all these things in the last (usable) 16k address space of things ..

abecedarius8 days ago

I felt the same way.

There was another downside, that the Applesoft BASIC license gave Microsoft a hostage which they later used to kill https://en.wikipedia.org/wiki/MacBASIC. Hard to tell how much it mattered that the Mac came with no user programmability, which had been expected for PCs till then.

Gracana8 days ago

Wow, somehow I didn't know about this. Now I'm angry, bundling an official Macintosh BASIC on every computer would have been huuuge for young-me.

jottinger8 days ago

The source code for this is apparently 8 years old: https://github.com/abagames/z80-repl

With that said, I love the idea.

wkjagt8 days ago

That makes me love it even more

NewsaHackO7 days ago

Yea, it doesn't have that... uninspired design to it, looks like the design of someone who actually created it themselves.

Jare7 days ago

Kenta Cho has created hundreds of games and toys over the past 20+ years. A lot of brilliant stuff.

networked8 days ago

Cool idea! The REPL needs to handle whitespace after an instruction with no operands:

  > ld c,20h
  0E 20   (7 cycles)
  > ld c,20h [<-- trailing space here]
  0E 20   (7 cycles)
  > nop
  00      (4 cycles)
  > nop [<-- trailing space here]
  unknown instruction: nop

I also wish I could select from the completion list with Tab without typing more of the instruction, fish-style. I.e., you press Tab to go through completion 1, completion 2, ..., completion N, completion 1 again with Enter to confirm your pick.

jhallenworld7 days ago

You should support symbols... For example, the monitor on my 6800 / 6809 Exorciser simulator has this (the "Offset at..." is printed when it fixes forward references):

    ~/exorsim$ exor -d
    % a 1000
    1000: start bra skip
    1002:  nop
    1003:  nop
    1004: skip ldx #10
    Offset at 1001 set to 02
    1007: loop dex
    1008:  nop
    1009:  bne loop
    100b:  rts
    100c: 
    % sy
    1007 loop
    1004 skip
    1000 start
    % u 1000
    1000: 20 02               BRA $1004
    1002: 01                  NOP
    1003: 01                  NOP
    1004: CE 00 0A            LDX #$000A
    1007: 09                  DEX
    1008: 01                  NOP
    1009: 26 FC               BNE $1007
    100B: 39                  RTS

What would be even better: "a" brings up a built-in mini-editor. When you exit, the code is assembled.

https://github.com/jhallen/exorsim

torutofu8 days ago

the fun part is how fast you can get back into Z80 muscle memory once the repl is giving you immediate feedback

Read the full thread on Hacker News →

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