Grammer2/src/readarc.z80

;RAM: 21+21n
;ARC, but no boundary: 114+21n
;Arc, on two pages: 21n+269
;Arc, on three pages: 21n+355
  or a
  jp z,ReadRAM    ;different routine in the App,
  out (6),a
  adc hl,bc
;  jr c,read_from_Arc_blocks   ;if you need this, you probably need a different routine. this will write on page 0.
  jp p,read_from_ARC_noboundary
read_from_Arc_blocks = $-ReadArcData+TSA
;If we make it here, we know that we cross a page boundary (or in one case, we just reach it and need to return on the next page).
;We will read in blocks to avoid checking page boundaries
;To do so, we first read up to 0x8000 - HL bytes
  xor a
  sbc hl,bc
  sub l \ ld l,a
  ld a,$80 \ sbc a,h \ ld h,a
  ;now we will subtract BC-HL  -> BC
  ld a,c \ sub l \ ld c,a
  ld a,b \ sbc a,h \ ld b,a
  push bc
  ld b,h
  ld c,l
  xor a \ sub l \ ld l,a
  ld a,$80 \ sbc a,h \ ld h,a
  ;now we read the first block
block_loop = $-ReadArcData+TSA
  ldir
  ;now we increment the page and continue reading from $4000
  in a,(6)
  inc a
  out (6),a
  ld h,40h
  pop bc
  ;if BC<$4000, just LDIR the rest
  ld a,b
  sub h
  jr c,read_from_RAM
  ld b,a
  push bc
  ld b,h
  ld c,l
  jp block_loop
read_from_ARC_noboundary  = $-ReadArcData+TSA
;  or a     ;already reset
  sbc hl,bc
read_from_RAM:
  ldir
  in a,(6)
  ld b,a
page_restore = $-ReadArcData+TSA+1
  ld a,0
  out (6),a
  ld a,b
  ld b,c
  ret