RC6502-Apple-1-Replica/software/examples/VDU-Notes/VDU Alphanumeric/vdu_alphanumeric.asm

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2020-01-21 23:34:29 +00:00
processor 6502
PAGECTR EQU $0030
ZPPTR EQU $0031
ZPPTR_LSB EQU $0031
ZPPTR_MSB EQU $0032
VDU_BASE EQU $8000
VDU_REG_0 EQU $8800
VDU_REG_1 EQU $8C00
; Main program
org $F000
INIT lda #$00
STA VDU_REG_0
STA VDU_REG_1
JSR CLEAR
JSR SET_MOTD
DONE JMP DONE
; Clear screen routine
CLEAR LDY #$80
LDX #$00
LDA #1
JSR CLEAR_RAM
RTS
CLEAR_RAM STA PAGECTR ;set pages to clear (1-255, 0 = 256)
STX ZPPTR_LSB ;set start address LSB
STY ZPPTR_MSB ;set start address MSB
LDA #$20 ;clear value -- could be anything
LDY #0 ;index
NEXT_BYTE STA (ZPPTR_LSB),y
INY
BNE NEXT_BYTE ;next location
DEC PAGECTR ;one less page to clear
BEQ CLEAR_DONE ;done
INC ZPPTR_MSB ;next page
BNE NEXT_BYTE ;more pages remaining
CLEAR_DONE RTS
; Write message to the display
SET_MOTD LDX #0
NEXT_CHAR LDA MESSAGE,x
CMP #0
BEQ MOTD_DONE
STA VDU_BASE,x
INX
JMP NEXT_CHAR
MOTD_DONE RTS
MESSAGE byte 12,03,36,35,30,32,20,16,04,15,0
; CPU initialization
org $FCCC ; Set initialization vectors
word INIT
word INIT