BASIC strings from assembler on Vic

[HarryP2]

I 'm working on a project for the Vic20 and want to be able to handle BASIC strings from assembler code. What addresses do I use for string-handling from an assembler program?

[HarryP2]

I just downloaded a ROM disassembly, but it has no addresses and is hard to navigate.

[HarryP2]

Never mind. I just found the Mapping the Vic PDF online. I was using an extra keyword that didn't appear in the listings.

[Mike]

The machine code surely will employ sort of an own memory allocation (i.e. at least running protected from the BASIC interpreter), so if you set aside a small buffer, at address AD say, you can pass strings between BASIC and machine code like thus:

BASIC -> ML: POKE AD,LEN(A$):IF LEN(A$)>0 THEN FOR T=1 TO LEN(A$):POKE AD+T,ASC(MID$(A$,T,1)):NEXT

ML -> BASIC: A$="":IF PEEK(AD)>0 THEN FOR T=1 TO PEEK(AD):A$=A$+CHR$(PEEK(AD+T)):NEXT

Address AD contains the length of the string, which might be zero in case of an empty string. Addresses AD+1 upto AD+LEN(A$) contain the string data itself. The buffer needs to be big enough to hold the longest strings you expect to handle. As BASIC strings are 255 characters maximum, a buffer of 256 bytes length (AD .. AD+255) is sufficient in any case.

[HarryP2]

I thank you for your reply, but I want to handle strings using BASIC ROM routines from ML. Maybe I should create my own routines to handle strings.

[Mike]

I thank you for your reply, but I want to handle strings using BASIC ROM routines from ML.

That was not 100% clear from your OP, but then so be it. The routines in the BASIC interpreter require quite some set-up to get them to do want you want, and any speed advantage compared to BASIC will be close to nil ...

Maybe I should create my own routines to handle strings.

... so that is pretty much what you will end up with anyway.

If you are so inclined, you might spare a look at my MG BROWSE utility. Besides the fact that it displays a scrolling soft 40 column text screen on the VIC-20, it reads in ASCII text from a file and formats it with word wrapping. I have included the source; the routines ".TFormat" and ".TWordWrap" do the heavy lifting. The source is supposed to be assembled with the inline assembler of Acorn BBC BASIC.

[HarryP2]

Thank you.

[wimoos]

Below is the ML-code behind he INSTR-function in WimBasic. The function searches a substring in a larger string, with an optional starting position.
The result is 0 when not found, non-zero is the location of the substring in the larger string.

Regards,

wimoos

Code: Select all

;
; Perform INSTR
;
LA776	JSR	$CEFA	; check bracket open
	JSR	$CD9E	; evaluate expression (A=($61), X=0, Y=$FF)
	BIT 	$0D		; check datatype
	BMI 	LC700	; branch if string
	JSR 	$D1AA	; convert to word in $64:$65 and in A:Y
	TAX			; check if >255
	BNE 	LA773	; 
	TYA			; may not be zero
	BEQ 	LA773
	PHA
	JSR $CEFD		; check comma
	JSR  $CD9E	; evaluate expression
	JSR  $CD8F	; check datatype to be string
	PLA
	TAX
	DEX
LC700	LDA $64		; save address of descriptor
	PHA
	LDA $65
	PHA
	TXA
	PHA
	JSR  $CEFD	; check comma
	JSR  $CD9E     	; evaluate expression 
	JSR  $D6A3 	; stack string
	JSR  $FE49	; save descriptor in $B7 and $BB:$BC
	TAX
	BEQ LA773
	PLA
	STA $A5
	PLA		
	TAY
	PLA
	JSR 	$D6AA	; discard string: length in A and address in $22:$23
	SEC            	 
	SBC  $B7       	 
	BCC  LA7E5     	 
	SBC  $A5       	 
	BCC  LA7E5     	 
	ADC  #$00      	 
	TAX            	 
	LDA  $A5       	 
	ADC  $22       	 
	STA  $22       	 
	BCC  LA7CB     	 
LA7C9	INC  $23       	 
LA7CB	INC  $A5
	LDY  $B7
LA7CF	TYA
	BEQ  LA7E0
	DEY
	LDA  ($22),Y
	CMP  ($BB),Y
	BEQ  LA7CF
	DEX
	BEQ  LA7E5
	INC  $22
	BNE  LA7CB
	BEQ  LA7C9	; branch always
;
LA7E5	LDY  #$00
	.BYTE $2C
LA7E0	LDY  $A5       	 
	JSR  $D3A2  ; return result
	JMP  $CEF7 ; check bracket close
;
LA773	JMP  $D248 ; ILLEGAL QUANTITY