Why appears here "hello world" 2x.

[funkheld]

Hi good afternoon.

Why appears here "hello world" 2x.

Thank you.

greeting

 

PROCESSOR 6502

INCLUDE "vcs.h"

 

ORG $F000         ; Start of "cart area" (see Atari memory map)

 

StartFrame:

lda #%00000010    ; Vertical sync is signaled by VSYNC's bit 1...

sta VSYNC

REPEAT 3          ; ...and lasts 3 scanlines

 sta WSYNC       ; (WSYNC write => wait for end of scanline)

REPEND

lda #0

sta VSYNC         ; Signal vertical sync by clearing the bit

 

PreparePlayfield:   ; We'll use the first VBLANK scanline for setup 

lda #124

sta COLUBK        ; Background color

lda #0 

lda #$1f          ; Playfield collor 

sta COLUPF

 

ldx #0            ; X will count visible scanlines, let's reset it

REPEAT 37         ; Wait until this (and the other 36) vertical blank

sta WSYNC     ; scanlines are finished

REPEND

lda #0            ; Vertical blank is done, we can "turn on" the beam

sta VBLANK

 

Scanline:

cpx #174          ; "HELLO WORLD" = (11 chars x 8 lines - 1) x 2 scanlines =

bcs ScanlineEnd   ; 174 (0 to 173). After that, skip drawing code

txa               ; We want each byte of the hello world phrase on 2 scanlines,

lsr               ; which means Y (bitmap counter) = X (scanline counter) / 2.

tay               ; For division by two we use (A-only) right-shift

lda Phrase,y      ; "Phrase,Y" = mem(Phrase+Y) (Y-th address after Phrase)

sta PF1           ; Put the value on PF bits 4-11 (0-3 is PF0, 12-15 is PF2)

ScanlineEnd:

sta WSYNC         ; Wait for scanline end

inx               ; Increase counter; repeat untill we got all kernel scanlines

cpx #191

bne Scanline

 

Overscan:

lda #%01000010    ; "turn off" the beam again...

sta VBLANK        ;

REPEAT 30         ; ...for 30 overscan scanlines...

sta WSYNC

REPEND

jmp StartFrame    ; ...and start it over!

 

Phrase:

.BYTE %00000000 ; H

.BYTE %01000010

.BYTE %01111110

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %00000000

.BYTE %00000000 ; E

.BYTE %01111110

.BYTE %01000000

.BYTE %01111100

.BYTE %01000000

.BYTE %01000000

.BYTE %01111110

.BYTE %00000000

.BYTE %00000000 ; L

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01111110

.BYTE %00000000

.BYTE %00000000 ; L

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01111110

.BYTE %00000000 ; O

.BYTE %00000000

.BYTE %00111100

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %00111100

.BYTE %00000000

.BYTE %00000000 ; white space

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000

.BYTE %00000000 ; W

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %01011010

.BYTE %00100100

.BYTE %00000000

.BYTE %00000000 ; O

.BYTE %00111100

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %01000010

.BYTE %00111100

.BYTE %00000000

.BYTE %00000000 ; R

.BYTE %01111100

.BYTE %01000010

.BYTE %01000010

.BYTE %01111100

.BYTE %01000100

.BYTE %01000010

.BYTE %00000000

.BYTE %00000000 ; L

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01000000

.BYTE %01111110

.BYTE %00000000

.BYTE %00000000 ; D

.BYTE %01111000

.BYTE %01000100

.BYTE %01000010

.BYTE %01000010

.BYTE %01000100

.BYTE %01111000

.BYTE %00000000 ; Last byte written to PF1 (important, ensures lower tip

; of letter "D" won't "bleeed")

 

ORG $FFFA             ; Cart config (so 6502 can start it up)

 

.WORD StartFrame      ; NMI

.WORD StartFrame      ; RESET

.WORD StartFrame      ; IRQ

 

  END

Edited March 25, 2017 by funkheld

[Just Jeff]

As designed, Atari will either repeat or reflect the left half of the playfield graphics on the right side. In this case, you are repeating the left side. Try storing a #1 to register CTRLPF and you will see your words reflected on the right half.

 

Note: many programs avoid repeating or reflecting the play field by changing those graphics registers with precise timing- after the beam has passed the left side but not yet reached the right side.

[funkheld]

hello, thanks.

 

A funny thing and what oll bring it?

 

greeting

[Just Jeff]

Hmm I'm not sure I understand your question. They did it to save money, memory, and machine time. The TIA only uses 3 bytes for the playfield registers this way. The games they imagined during the design- Combat, Pong, etc use reflected playfields for example.