Assembly: How to save RAM, and how to print RAM to the screen

rje · Post by **rje** » Sat Dec 25, 2021 3:29 pm

Thank you Stefan — I didn’t know that.

I had the ADC in mind, if I stored 1 in A, then I’d be incrementing with carry. Then I’d have to BCS or whatever that’s called, which is still messy I suppose.

rje · Post by **rje** » Sat Dec 25, 2021 3:40 pm

Found hints of this on the infowebs...

But I may be very wrong.

clc

Lda #01

adc >my-address

adc <my-address

Its probably really slow, right? 4 clocks each for the ADC $$$$. Or there’s a fundamental flaw in my logic.

rje · Post by **rje** » Sat Dec 25, 2021 3:44 pm

This makes me think about SWEET16

Stefan · Post by **Stefan** » Sat Dec 25, 2021 4:10 pm

I guess you could use ADC, but it would be more code and slower.

Assuming you are using r0 as zero page vector, this might work (not tested):

ldx #<$8000

ldy #>$8000

stx r0

sty r0+1

loop:

clc

lda r0

adc #1

sta r0

lda r0+1

adc #0 ;Adding carry

sta r0+1

lda (r0) ;Indirect addressing mode without Y, not supported by original 6502

beq end

jsr $ffd2 ;Print char

bra loop

end:

rts

Ed Minchau · Post by **Ed Minchau** » Sat Dec 25, 2021 10:31 pm

On 12/25/2021 at 8:40 AM, rje said:

Found hints of this on the infowebs...

But I may be very wrong.

clc

Lda #01

adc >my-address

adc <my-address

Its probably really slow, right? 4 clocks each for the ADC $$$$. Or there’s a fundamental flaw in my logic.

You're just looking to increment a 16 bit index spread across two bytes. The INC and INX and INY operations set the Z flag if the result is zero and sets the N flag if the most significant bit of the result is 1.

So, if your 16 bit value is stored in say zero page addresses 7E (low byte) and 7F (high byte) then

INC 7E

BNE# 02

INC 7F

And @ZeroByte is correct, the easiest way to save something from VRAM to file is to copy it to low RAM and save it from there. Or if it is a big file, copy it to consecutive banks of banked RAM and save it from there.

ZeroByte · Post by **ZeroByte** » Sun Dec 26, 2021 2:45 am

On 12/23/2021 at 9:53 PM, Greg King said:

That's BASIC's way of doing it. In Assembly, SETLFS doesn't take anything in the accumulator for LOAD/SAVE calls. And, the "VRAM bank + 2" is put into the accumulator of the LOAD call (i.e., the 17-bit VRAM address is given directly to the LOAD call).

I knew better than that - it's the .A value of LOAD, not SETLFS. Whoops. (fixed my post)

Thanks for pointing it out.