(sccz80) Gameboy z80 support

[suborb]

Just leaving this here for thoughts. We've managed to support the 8080 without too much trouble, so lets turn to the gbz80:

Missing:

• No index registers
• No alternate registers
• No block instructions
• No in/out (memory mapped io instead)
• No 0xed instruction page (so ld rr,(nnnn), ld (nnnn),rr, sbc hl,rr, adc hl,rr are absent)
• No ld hl,(nnnn), ld (nnnn),hl
• No ex (sp),hl, ex de,hl
• No djnz
• No sign or parity flags
• POP AF doesn't preserve low 4 bits of F (see above)

Extra (useful) instructions:

• ld hl,sp+dd
• add sp,dd
• ld a,(hl+)
• ld (hl+),a
• ld a,(hl-)
• ld (hl-),a

Movements:

• Opcodes moved: reti, ld a,(nnnn), ld (nnnn),a

So it's mostly 8080, the main issues at first glance are the missing ex instructions and ld (nnnn),hl. ld hl,(nnnn) is fairly easy to emulate if we can trash a.

Library support should be from gbdk, there's an up-to-date version here: https://github.com/andreasjhkarlsson/gbdk-n

[pauloscustodio]

Shall we use -mgbz80 as the machine specifier option? (to be the same in z80asm and sccz80)

[suborb]

Thanks for picking up the z80asm side Paulo.

That’s the value that I’ve chosen - matches the one used by sdcc.

[suborb]

@pauloscustodio I think djnz has an off by one issue:

Source:

flags_again:
        push    hl              ;save fmt
        ld      b,5
        ld      hl,flags
flag_loop:
        cp      (hl)
        inc     hl
        jr      nz,no_flag
; We've found a flag
        ld      a,(hl)          ;pick up flags
        or      c
        ld      c,a
        pop     hl              ;get fmt back
        ld      a,(hl)          ;pick up next character
        inc     hl
        jr      flags_again
flags:
        defb    '-', 0x01
        defb    '+', 0x02
        defb    ' ', 0x08
        defb    '#', 0x10
        defb    '0', 0x04
no_flag:
        inc     hl
        djnz    flag_loop

Assembled to:

flags_again:
                    push    hl                              ;[0155] e5
                    ld      b,$05                           ;[0156] 06 05
                    ld      hl,$0167                        ;[0158] 21 67 01
flag_loop:
                    cp      (hl)                            ;[015b] be
                    inc     hl                              ;[015c] 23
                    jr      nz,no_flag                      ;[015d] 20 12
                    ld      a,(hl)                          ;[015f] 7e
                    or      c                               ;[0160] b1
                    ld      c,a                             ;[0161] 4f
                    pop     hl                              ;[0162] e1
                    ld      a,(hl)                          ;[0163] 7e
                    inc     hl                              ;[0164] 23
                    jr      flags_again                     ;[0165] 18 ee
flags:
                    dec     l                               ;[0167] 2d
                    ld      bc,$022b                        ;[0168] 01 2b 02
                    jr      nz,$0175                        ;[016b] 20 08
                    inc     hl                              ;[016d] 23
                    stop                                    ;[016e] 10
                    jr      nc,$0175                        ;[016f] 30 04
no_flag:
                    inc     hl                              ;[0171] 23
                    dec     b                               ;[0172] 05
                    jr      nz,$015c                        ;[0173] 20 e7

[suborb]

Status update:

The basics are working:

• sccz80 generates the correct I/O code for 8 bit __sfr
• sccz80 uses the new stack routines
• +test has a -clib=gbz80 target with pretty much the same library as an 8080 port.
• A fair amount of the test-suite is running
• z80asm does emulation for ldir and friends

Remaining:

• Half + carry flags for add hl,sp, ld hl,sp+nnn
• A Gameboy crt0
• Import the standard GBDK library
• Example
• Figure out sdcc interop will work (sdcc uses reversed de+hl)
• sdcc crt0 routines

[suborb]

I've now got galaxy.c from gbdk compiling and mostly working.

I think sdcc interop goes away at this stage, getting a whole bunch of errors along the lines of:

Function canAssign not initialized in port structure
Function notUsed not initialized in port structure
Caught signal 11: SIGSEGV

Which will need to be investigated later.

• The not-initialised messages are due to the z80 rules being applied in gbz80 mode.
• Seg fault is probably due to #1279

[suborb]

Closing, will reopen as a separate issue with remaining items