gp2io/sendbyte.s
Charles Mangin 7c5943956b added Merlin source for main routines
Merlin32 compatible source for readbyte, sendbyte, and bufferedio
routines
2017-04-10 12:16:47 -04:00

28 lines
1.3 KiB
ArmAsm

ORG $0300
ANN0HI EQU $C059
ANN0LO EQU $C058
ANN1HI EQU $C05B
ANN1LO EQU $C05A
BYTETOSEND EQU $EF
SENDBYTE LDY #$08 ; load 8 for a full byte, loop counter
STA BYTETOSEND ; put byte in zero page for safe keeping
RTS STA ANN0HI ; annunciator 0 high, RTS
SENDBIT ROL BYTETOSEND ; rotate byte left, high/MSB out to carry (sendbit)
BCC SHORTLOOP ; "branch on carry clear" - if carry/bit = 0, goto #311 (short loop)
LONGLOOP LDX #$14 ; if carry/bit = 1, load X with 20 (long loop)
JMP SETANN1 ; jump over SHORTLOOP
SHORTLOOP LDX #$0A ; if carry/bit = 0, load X with 10 (short loop)
SETANN1 STA ANN1HI ; set annunciator 1 HIGH
COUNTDOWN DEX ; decrement, countdown to setting ANN1 low (COUNTDOWN)
BNE COUNTDOWN ; if X > 0, keep counting (goto COUNTDOWN)
STA ANN1LO ; if done, set ANN1 LOW, countdown to getting next bit
LDX #$05 ; reset counter for short "reset" transition. This lets the GP2IO stage the bit before the next interrupt
COUNTDOWN2 DEX ; count down again (COUNTDOWN2)
BNE COUNTDOWN2 ; if X > 0, keep counting (goto COUNTDOWN2)
DEY ; decrement y, countdown bits sent
BNE SENDBIT ; if y > 0, next bit (SENDBIT)
RTSOFF STA ANN0LO ; annunciator 0 low, sending RTS OFF
RTS ; return