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base:petscii_to_screencode

PETSCII to screencode conversion

By Mace

While creating my own routine to display a floppy directory, I stumbled accross the awkward difference between ASCII/PETSCII codes and screencodes. In *SCII, the letter A is value 65, while on the screen it is value 1. Yet, simply subtracting 64 from all *SCII codes doesn't work. An easy way of converting would be to create a table, but that takes quite some space for something that is not that hard to code. Simply said, you have to divide the *SCII code in blocks and for each block apply a certain calculation to get the screencode.

It could look something like this:

		// A = *SCII-code

		cmp #$20		// if A<32 then...
		bcc ddRev

		cmp #$60		// if A<96 then...
		bcc dd1

		cmp #$80		// if A<128 then...
		bcc dd2

		cmp #$a0		// if A<160 then...
		bcc dd3

		cmp #$c0		// if A<192 then...
		bcc dd4

		cmp #$ff		// if A<255 then...
		bcc ddRev

		lda #$7e		// A=255, then A=126
		bne ddEnd

	dd2:	and #$5f		// if A=96..127 then strip bits 5 and 7
		bne ddEnd

	dd3:	ora #$40		// if A=128..159, then set bit 6
		bne ddEnd

	dd4:	eor #$c0		// if A=160..191 then flip bits 6 and 7
		bne ddEnd

	dd1:	and #$3f		// if A=32..95 then strip bits 6 and 7
		bpl ddEnd		// <- you could also do .byte $0c here

	ddRev:	eor #$80		// flip bit 7 (reverse on when off and vice versa)
	ddEnd:
		// screencode is now in accu

If you would like to use a table after all, you could use this one:

tab_petscii2screencode:
										;PETSCII RANGE
	.byte $80,$81,$82,$83,$84,$85,$86,$87, $88,$89,$8a,$8b,$8c,$8d,$8e,$8f	;$00-...
	.byte $90,$91,$92,$93,$94,$95,$96,$97, $98,$99,$9a,$9b,$9c,$9d,$9e,$9f	;...-$1f
	.byte $20,$21,$22,$23,$24,$25,$26,$27, $28,$29,$2a,$2b,$2c,$2d,$2e,$2f	;$20-...
 	.byte $30,$31,$32,$33,$34,$35,$36,$37, $38,$39,$3a,$3b,$3c,$3d,$3e,$3f	;...-$3f
	.byte $00,$01,$02,$03,$04,$05,$06,$07, $08,$09,$0a,$0b,$0c,$0d,$0e,$0f	;$40-...
 	.byte $10,$11,$12,$13,$14,$15,$16,$17, $18,$19,$1a,$1b,$1c,$1d,$1e,$1f	;...-$5f
	.byte $40,$41,$42,$43,$44,$45,$46,$47, $48,$49,$4a,$4b,$4c,$4d,$4e,$4f	;$60-...
 	.byte $50,$51,$52,$53,$54,$55,$56,$57, $58,$59,$5a,$5b,$5c,$5d,$5e,$5f	;...-$7f
	.byte $c0,$c1,$c2,$c3,$c4,$c5,$c6,$c7, $c8,$c9,$ca,$cb,$cc,$cd,$ce,$cf	;$80-...
 	.byte $d0,$d1,$d2,$d3,$d4,$d5,$d6,$d7, $d8,$d9,$da,$db,$dc,$dd,$de,$df	;...-$9f
	.byte $60,$61,$62,$63,$64,$65,$66,$67, $68,$69,$6a,$6b,$6c,$6d,$6e,$6f	;$a0-...
 	.byte $70,$71,$72,$73,$74,$75,$76,$77, $78,$79,$7a,$7b,$7c,$7d,$7e,$7f	;...-$bf
	.byte $00,$01,$02,$03,$04,$05,$06,$07, $08,$09,$0a,$0b,$0c,$0d,$0e,$0f	;$c0-...
 	.byte $10,$11,$12,$13,$14,$15,$16,$17, $18,$19,$1a,$1b,$1c,$1d,$1e,$1f	;...-$df
	.byte $60,$61,$62,$63,$64,$65,$66,$67, $68,$69,$6a,$6b,$6c,$6d,$6e,$6f	;$e0-...
 	.byte $70,$71,$72,$73,$74,$75,$76,$77, $78,$79,$7a,$7b,$7c,$7d,$7e,$5e	;...-$ff
base/petscii_to_screencode.txt · Last modified: 2015-04-17 04:33 (external edit)