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rst $28
.db $0A,$45 ;$450A aka _puts
$28:
jp 2810
;hl = pointer to text label
$2810:
push hl ;sp -2
push hl ;-4
push hl ;-6
push af ;-8 save af
push bc ;-10 save bc
push de ;-12 save de
push hl ;-14 save hl
ld hl,000f ;15
add hl,sp
;stack starts at $FFFF and counts down, so this is like:
;sub sp,15
ld d,(hl) ;hl points to the upper byte of (pc) address that was pushed onto the stack with our reset call
dec hl ;move to lower byte
ld e,(hl) ;
inc de ;skip over the address of the bcall
inc de ;de now points to the code just after bcall(_puts)
ld (hl),e ;essentially change our return address to just after the bcall
inc hl
ld (hl),d
in a,($06)
ld c,a
dec hl
dec hl
dec hl ;points LSB of first push hl
ld (hl),a ;replace with value of in a,($06)
dec hl
ld (hl),$28
dec hl
ld (hl),$e7 ;change 2nd push hl to 28e7 location
dec de
ld a,(de)
ld h,a ;MSB of our bcall ($45)
dec de
ld a,(de)
ld l,a ;LSB of our bcall ($0A)
;hl = pointer to bcall
ld b,0 ;bc = whatever in a,($06) means
bit 7,h ;bit 7 is not on
jr nz,$2846 ;if it is on, reset it
bit 6,h ;bit 7 is off, check if bit 6 is on/off
jr z,$284c :if it is off, do something. else reset it
$2840:
res 6,h
ld a,$1b ;27
jr $2857
$2846:
res 7,h
ld a,$1f ;31
jr $2857
$284c:
in a,($06)
and $1f ;make sure not >31 cause below has 32 bytes
ld de,$8230 ;ti83p.inc defines this as "baseAppBrTab"
add a,e ;add the offset
ld e,a
ld a,(de) ;store value at offset to a
ld b,a ;store into b
$2857: ;bit 7 or 6 was reset, or both are already reset
out ($06),a ;output whatever those values mean
ld de,$4000
add hl,de ;why do we do this? we reset bit 6 then effectively add it right back
ld e,l
ld d,h ;load bcall location into de
ld hl,$0008
add hl,sp ;point to the stack position of 3rd push hl
ld a,(de) ;load LSB from the location pointed to at bcall
ld (hl),a ;store it in stack
inc de
inc hl
ld a,(de)
ld (hl),a ;load from the LUT and store where it points to into stack
inc de ;again not sure what the purpose of this is
ld a,b
or a
jr z,$2874
ld a,(de)
neg
add a,b
jr $2879
$2874:
or a
jr nz,$2879
ld a,c
$2879:
out ($06),a ;this seems to do something to the bcall lookup table?
pop hl ;recall all our values (these should be unchanged by all the stack manipulation)
pop de
pop bc
pop af
ret ;jump to bcall
I was initially trying to find where the TI stores the sprites for the large font, but it turned out not to be as simple as i thought. But anyway, that is another subject altogether (that led me to many more questions!) and i think it would be clearer if i just made another topic on that.
But anyway, if it isn't too much trouble, could someone explain what exactly is going on here?
