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49 lines
1020 B
NASM
49 lines
1020 B
NASM
;;
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;; Read input from the keyboard, and echo to console.
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;;
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IOBASE = $8800
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IOSTATUS = IOBASE + 1
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IOCMD = IOBASE + 2
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IOCTRL = IOBASE + 3
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.segment "CODE"
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.org $C000
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START: CLI
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LDA #$09
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STA IOCMD ; Set command status
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LDA #$1A
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STA IOCTRL ; 0 stop bits, 8 bit word, 2400 baud
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;;
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;; Infinite idle loop, waiting for interrupt.
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;;
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IDLE: LDA #$FF
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TAX
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IDLE1: DEX
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BNE IDLE1
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JMP IDLE
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;;
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;; IRQ handler
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;;
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IRQ: LDA IOBASE ; Get the character in the ACIA.
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PHA ; Save accumulator
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ECHO1: LDA IOSTATUS ; Read the ACIA status
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AND #$10 ; Is the tx register empty?
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BEQ ECHO1 ; No, wait for it to empty
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PLA ; Otherwise, load saved accumulator,
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STA IOBASE ; write to output,
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RTI ; and return
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; system vectors
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.segment "VECTORS"
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.org $FFFA
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.word IRQ ; NMI vector
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.word START ; RESET vector
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.word IRQ ; IRQ vector
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