assembly - Why are characters being offset by 0x40 on my Commodore 64 emulator? -

i have 6502 code print string screen memory after clearing screen. unfortunately if print string, example "hello world", come out in garbled characters. i've found because upper case characters start @ 0x01, not 0x41 thought petscii codes here.

i can fix subtracting 0x40 string, other letters incorrect, example, spaces. i'm not sure why character generator turning 0x01 character 'a' , not 0x41. turns 0x41 inverted spade sign (like on deck of cards) , above seems border characters , weird symbols.

after looking around while found quote on wikipedia page petscii seemed state problem i'm trying solve, i'm not sure how fix , can't find information anywhere...

the actual character generator rom used different set of assignments. example, display characters "@abc" on screen directly pokeing screen memory, 1 poke decimal values 0, 1, 2, , 3 rather 64, 65, 66, , 67.

i running on vice x64 emulator on mac os x, , i'm assembling os x port of 64tass.

this assembly code without subtracting 0x40:

*=$c000  border = $d020 inner = $d021  start   lda #0         sta border         lda #0         sta inner          jsr clear         jsr string  loop             jmp loop   clear   ; clear screen         lda #$00         tax         lda #$20 clrloop          sta $0400, x ; clear each memory "row"         sta $0500, x         sta $0600, x         sta $0700, x         dex         bne clrloop  ; clear if x != 0         rts  string  ; load string         ldx #$0 strloop lda hello, x ; load each byte in turn         cmp #0       ; if reached null byte, break         beq strexit         sta $0400, x         inx         jmp strloop strexit rts   hello   .text "hello world"         .byte 0 

here screenshot of output

thanks in comments!

side note others

you can set row , column chrout output setting cursor position plot

http://sta.c64.org/cbm64krnfunc.html

you're possibly writing ascii codes screen memory directly, that's why it's offset $40.

to have them in petscii, need add "-a" option 64tass. alone not enough. example offset $c0 (uppercase petscii letters) now. changing text lowercase still gives $40 offset (lowercase petscii).

you need write "screen" codes screen. fortunately there's built in conversion in 64tass if this:

        .enc screen            ; switch screen code encoding hello   .text "hello world"         .byte 0         .enc none 

but remember "@" 0 in screen code, it'll termitate loop. text in lower case, default font uppercase it'll end uppercase. set $d018 $16 switch lower case font, it'll match write.

a proper petscii example be:

    *=$c000      lda #0     sta $d020 ; border     sta $d021 ; background      ldx #0 lp  lda hello,x     beq end     jsr $ffd2 ;print character     inx     bne lp end rts  hello .null "{clr}{swlc}hello world" 

compile not old 64tass translates "{clr}" , "{swlc}" control codes 147 , 14. , don't forget "-a" switch enable unicode support, otherwise assembler won't translation of string , copy verbatim (as raw bytes).