ca65 V2.17 - Git 582aa41
Main file   : branch_offset_calc.ca65
Current file: branch_offset_calc.ca65

000000r 1               ;
000000r 1               ; ACORN System 1 Applications: Branch Offset Calculator
000000r 1               ;
000000r 1               ; From the Acorn System 1 User's Manual
000000r 1               ;
000000r 1               ; (Chris Oddy January 2022)
000000r 1               ;
000000r 1               ; entry point $0200
000000r 1               ;
000000r 1               		.setcpu			"6502"
000000r 1               		.listbytes		unlimited
000000r 1               ;
000000r 1               D		:=		$10
000000r 1               MESSL	:=		$20
000000r 1               MESSH	:=		$21
000000r 1               FROMH	:=		$22
000000r 1               FROML	:=		$23
000000r 1               TOL		:=		$24
000000r 1               TOH		:=		$25
000000r 1               DISPLAY	:=		$FE0C		; scan the display
000000r 1               RDHEXTD	:=		$FE60		; right double hex to display
000000r 1               QDATFET	:=		$FE88		; quad data fetch
000000r 1               RESTART	:=		$FF04		; Monitor reentry point
000000r 1               ;
000000r 1               		.org			$0200
000200  1               ;
000200  1  D8           OFFSET:	cld
000201  1  A9 02        AGAIN:	lda		#$02
000203  1  85 21        		sta		MESSH		; initialise message pointer
000205  1  84 22        		sty		FROMH		; setup prompt
000207  1  84 23        		sty		FROML
000209  1  A2 22        		ldx		#FROMH
00020B  1  20 88 FE     		jsr		QDATFET		; fetch first address
00020E  1  A9 11        		lda		#$11		; set up 2nd prompt
000210  1  85 24        		sta		TOL
000212  1  85 25        		sta		TOH
000214  1  A2 24        		ldx		#TOL
000216  1  20 88 FE     		jsr		QDATFET		; fetch second adress
000219  1  A5 22        		lda		FROMH		; offset to make overlength easy
00021B  1  E9 7E        		sbc		#$7E		; carry known set by QDATFET
00021D  1  85 22        		sta		FROMH
00021F  1  B0 03        		bcs		HSUB		; don't set the carry again
000221  1  C6 23        		dec		FROML
000223  1  38           		sec
000224  1  A5 24        HSUB:	lda		TOL			; calculate the length
000226  1  E5 22        		sbc		FROMH
000228  1  AA           		tax
000229  1  A5 25        		lda		TOH
00022B  1  E5 23        		sbc		FROML
00022D  1  D0 0E        		bne		TOOFAR
00022F  1  A9 55        		lda		#$55
000231  1  20 48 02     		jsr		MESSAGE		; print out
000234  1  8A           		txa
000235  1  49 80        		eor		#$80		; complement top bit because of the offset applied
000237  1  20 60 FE     		jsr		RDHEXTD		; print out answer, over writing the
00023A  1  4C 04 FF     		jmp		RESTART		; finished
00023D  1  A9 5B        TOOFAR:	lda		#$5B		; whoops
00023F  1  20 48 02     		jsr		MESSAGE		; tell the programmer that its wrong
000242  1  20 0C FE     		jsr		DISPLAY
000245  1  4C 01 02     		jmp		AGAIN
000248  1  85 20        MESSAGE:sta		MESSL		; message described by A
00024A  1  A0 07        		ldy		#$07		; eight bytes of data to display
00024C  1  B1 20        LOOP:	lda		(MESSL),y	; fetch them
00024E  1  99 10 00     		sta		D,y
000251  1  88           		dey
000252  1  10 F8        		bpl		LOOP
000254  1  60           		rts
000255  1  53 71 71 6D  		.byte	$53,$71,$71,$6D,$79,$F8,$78,$5C
000259  1  79 F8 78 5C  
00025D  1  5C 00 71 5C  		.byte	$5C,$00,$71,$5C,$50,$00
000261  1  50 00        
000262  1