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ExtendeD
PostPosted: Mon Oct 11, 2004 9:15 am 
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Posts: 6
I am currently trying to debug the PPU core of my NES emulator, and using the debug version of fceu, i found something strange about reading VRAM.
I thought that:
LDA #$08
STA $2006
LDA #$81
STA $2006
LDA $2007
would return in register A the byte of VRAM located at adress $881 (as specified by the two writes in register $2006). But I dumped the PPU memory and looked at address $881, and I found out that the byte returned in A with the code above was not the byte I found by hand.
Is there something I forgot (Im sure I didnt make any mistake looking at the byte located at $881 in PPU mem) with reading the PPU with register $2007?
thanks by advance ;)
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GARTHWILSON
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PostPosted: Mon Oct 11, 2004 2:47 pm 
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Each address holds only 8 bits. You can't store a 16-bit address in one 8-bit location. Do:

Code:
     LDA   #$81   ; Put low byte of address
     STA   $26    ; in 0026
     LDA   #8     ; and high byte of address
     STA   $27    ; in 0027.
     LDA   (26)   ; Read address 881, as pointed
                  ; to by $0026 and 0027.

For an indirect, the pair of bytes that points to the address must be in zero page. Notice also that the low byte goes in the low address of a pair, and the high byte goes in the high address. This allowed the designers to make the processor faster.

Each of the instructions above takes two bytes. The LDA#'s take 2 clocks each, the STA's take 3 clocks each, and the LDA(indirect) takes 5 clocks.
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8BIT
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PostPosted: Mon Oct 11, 2004 2:47 pm 
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Location: Sacramento, CA
I have not used the NES before.
Have you tried this?

Code:
LDA #$81
STA $2006
LDA #$08
STA $2006
LDA $2007


Just a thought.

Daryl
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GARTHWILSON
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PostPosted: Mon Oct 11, 2004 2:58 pm 
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If you're right Daryl, maybe you know something I don't. He used a couple of acronyms I'm not familiar with. I assumed VRAM was video RAM but not separate from the processor's own bus. In all my years of fooling with this stuff, I've never come across the term "PPU" or "fceu". But if this VRAM were some sort of IC that only takes a few addresses in the 6502's memory map but has a load of its own memory internally, I doubt you'd write to the same address for both high byte and low byte.
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ExtendeD
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PostPosted: Mon Oct 11, 2004 3:00 pm 
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Hi 8BIT,
yes I have tried this code. On the NES when you write at adress $2006, the first write specifies the upper byte of the 16bits adress to access with $2007, and the second write the lower byte.
So:
LDA #$08
STA $2006
LDA #$81
STA $2006
LDA $2007

specifies that the next access to $2007 will read/write at adress $0881 of PPU (LDA $2007 -> reads the PPU adress specified with the two writes to $2006 above ($881), and stores the byte found at this location in A).
the problem is when I debugged a NES game (with a bug-free nes emulator/debuger), I found out that the byte stored in A after this piece of code wasen't the one I found at address $881 when I dumped the PPU memory at this moment.
(anyway I think you should have already programmed on the NES to answer this question :) :))
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ExtendeD
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PostPosted: Mon Oct 11, 2004 3:03 pm 
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PPU -> Picture Processing Unit, a chip of the NES independed of the CPU, wich treats with display/io.
fceu -> a famous nes emulator/debugger
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BitWise
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PostPosted: Mon Oct 11, 2004 8:14 pm 
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The NESTech document warns that first read after setting the VRAM address returns invalid data.
Code:
  In a real NES, reading/writing VRAM should only be attempted during
VBlank. Many smaller ROMs have CHR-RAM for the Pattern Tables. In this
case, you won't be able to write into this memory.

  Writing to VRAM
  ---------------
  1) Write upper address byte into $2006
  2) Write lower address byte into $2006
  3) Write data into $2007. After each write, the address will auto-
     increment by 1, or 32 (if Bit #2 of $2000 is 1).

  Reading from VRAM
  -----------------
  1) Write upper address byte into $2006
  2) Write lower address byte into $2006
  3) Read data from $2007. The first read from $2007 is invalid, and
     therefore should be taken care of before actual data is read.
  4) Read data from $2007. From here on, after each read, the address will
     auto-increment by 1, or 32 (if Bit #2 of $2000 is 1).

I found the full NESTech document here http://patpend.net/technical/nes/nestech.doc.txt

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ExtendeD
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PostPosted: Sun Oct 17, 2004 4:32 pm 
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Thanks a lot BitWise, that's exactly what I was looking for :)
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Memblers
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PostPosted: Tue Oct 19, 2004 4:07 am 
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Posts: 64
Location: Indianapolis
The data on the first read will be invalid because you're reading the (empty) buffer, after that it's fine because the buffer will be pre-loaded. What's strange though, I believe that doesn't happen when you read palette RAM (palette memory is internal to the PPU).

Also with the $2006 being a 2-write register, if it's in an unknown state you can reset it by reading $2002.

More docs on my site: http://nesdev.parodius.com
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