Error #6 in Ppu Access in Blargg's PPU Test

Error #6 in Ppu Access in Blargg's PPU Test
by Anes on 2011-06-08 (#79458)

Im asking for help since vram_access.nes throw me error #6.
According to blargg doc the error is

"Palette read should also read VRAM into read buffer"

Im using a buffer for 2007 reads, but not for Pallete addr read.
Can somebody help me?

Re: Error #6 in Ppu Access in Blargg's PPU Test
by cpow on 2011-06-08 (#79459)

Anes wrote:

Usually a quick peek at the source code for those tests will tell you what the test is expecting to be "truth" and then you should be able to step through your code while executing the test and inspect your variation of "truth".

Have you looked at the test source code yet?

by Nessie on 2011-06-08 (#79461)

Reading $3F00-$3FFF should work just like $3000-$3EFF, i.e. a nametable byte should be put in the VRAM buffer as usual.
However, the value that is finally returned to the CPU will not be the what was previously in the VRAM buffer. Instead, it will be a value from the palette.

Something like this:
Code:
read_2007:
previously_buffered = vram_buffer
vram_buffer = read_ppu_memory(address)
if address >= $3F00
return read_palette(address)
else
return previously_buffered

by Anes on 2011-06-09 (#79491)

Code:
read_2007:
previously_buffered = vram_buffer
vram_buffer = read_ppu_memory(address)
if address >= $3F00
return read_palette(address)
else
return previously_buffered

Thanks for your code, but it didn't work for me I dont know why.
It's supoussed that the code explain more or less how the buffer works, but i dont get it.
Can somebody explain me how the buffer works?

by cpow on 2011-06-09 (#79492)

Okay, here's the relevant code for the test you're failing...I annotated with comments.

Code:
; Set VRAM addr to $2f00 + A
; Preserved: A, X, Y
set_vram_pos:
pha
lda #$2f
sta $2006
pla
sta $2006
rts

...

lda #6;) Palette read should also read VRAM into read buffer
sta result
lda #$12
jsr set_vram_pos ; sets VRAM address to $2f12
lda #$9a
sta $2007 ; stores $9a at $2f12, increments VRAM address to $2f13
lda $2007 ; returns VRAM read-buffered data, and reads from $2f13 (which will not contain $9a so you'd know if you read the wrong byte back or didn't do the buffering correctly)
lda #$3f ;
sta $2006 ;
lda #$12 ;
sta $2006 ; the above lines set VRAM address to $3f12
lda $2007 ; fills buffer with VRAM hidden by palette (in this case, $9a from $2f12, because $2000-$2fff are mirrored at $3000-$3fff)
lda #$13 ; change back to non-palette addr to enable buffer
jsr set_vram_pos ; set VRAM address to $2f13
lda $2007 ; read from VRAM, should return $9a which is in the buffer, perform a read from VRAM $2f13, and increment VRAM address
cmp #$9a
jsr error_if_ne ; if you didn't get $9a from that VRAM read you fail.