mirror of
https://github.com/tebl/RC6502-Apple-1-Replica.git
synced 2024-12-22 12:29:45 +00:00
66 lines
1.4 KiB
NASM
66 lines
1.4 KiB
NASM
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processor 6502
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PAGECTR EQU $0030
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ZPPTR EQU $0031
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ZPPTR_LSB EQU $0031
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ZPPTR_MSB EQU $0032
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VDU_BASE EQU $8000
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VDU_REG_0 EQU $8800
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VDU_REG_1 EQU $8C00
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; Main program
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org $F000
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INIT lda #$00
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STA VDU_REG_0
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STA VDU_REG_1
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JSR CLEAR
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JSR SET_MOTD
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DONE JMP DONE
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; Clear screen routine
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CLEAR LDY #$80
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LDX #$00
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LDA #1
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JSR CLEAR_RAM
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RTS
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CLEAR_RAM STA PAGECTR ;set pages to clear (1-255, 0 = 256)
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STX ZPPTR_LSB ;set start address LSB
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STY ZPPTR_MSB ;set start address MSB
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LDA #$20 ;clear value -- could be anything
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LDY #0 ;index
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NEXT_BYTE STA (ZPPTR_LSB),y
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INY
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BNE NEXT_BYTE ;next location
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DEC PAGECTR ;one less page to clear
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BEQ CLEAR_DONE ;done
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INC ZPPTR_MSB ;next page
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BNE NEXT_BYTE ;more pages remaining
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CLEAR_DONE RTS
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; Write message to the display
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SET_MOTD LDX #0
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NEXT_CHAR LDA MESSAGE,x
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CMP #0
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BEQ MOTD_DONE
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STA VDU_BASE,x
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INX
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JMP NEXT_CHAR
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MOTD_DONE RTS
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MESSAGE byte 12,03,36,35,30,32,20,16,04,15,0
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; CPU initialization
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org $FCCC ; Set initialization vectors
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word INIT
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word INIT
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