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Main - ROM Hacking - If you've ever programmed the NES... New thread | New reply


RetroRain
Posted on 03-08-11 04:39 AM Link | Quote | ID: 140050


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A question for those who have ever programmed the NES. Do you need to look at NES documents, or are you able to just program without looking at anything else?

I find that I have to look at a the document nestech.txt, because I can't remember everything, but I was just wondering for those of you who have made small NES demos or homebrews, or have done any kind of NES programming from scratch, who are experienced programmers, if you can program the NES without looking at the specs. It wouldn't be considered "amateur-ish" to have to look right?

What made me ask this question?

Just looking at the asm code from programmers who made small NES demos. I feel like that maybe I'm not experienced enough because I have to always look at certain documentation.

For example, while I am familiar with how the NES operates, and I know the basics of C, when I was following a NES tutorial to make a demo, there were commands that I never recognized, like:

.org
.dw
.bank

Are these considered ASM or NES-specific commands?

And I never used the stack. Is it possible to get by without it? Probably because I think documentation on it doesn't answer my questions.

How many set of numbers can you place on a stack, or is it unlimited, because the stack is not a RAM address right? I've seen no indication that it is. All I know is that it allows you to place numbers on it temporarily so you can quickly use them again.

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NovaYoshi
Posted on 03-08-11 04:50 AM Link | Quote | ID: 140051


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Just skimming over the original post, and I probably did a terrible job at explaining stuff, but...

Posted by RetroRain
NOTE: I posted this in ROM hacking instead of Programming because I believe it relates more to the NES and ROMs

A question for those who have ever programmed the NES. Do you need to look at NES documents, or are you able to just program without looking at anything else?
I find that I have to look at a the document nestech.txt, because I can't remember everything, but I was just wondering for those of you who have made small NES demos or homebrews, or have done any kind of NES programming from scratch, who are experienced programmers, if you can program the NES without looking at the specs. It wouldn't be considered "amateur-ish" to have to look right?

I look all the time, but decreasingly so. I started to remember the register names (Easier than memorizing addresses) instead of having to look them up each time, but I still have to look up the PPUCTRL flags, PPUMASK flags, and OAM flags each time I deal with them.

Posted by RetroRain

For example, while I am familiar with how the NES operates, and I know the basics of C, when I was following a NES tutorial to make a demo, there were commands that I never recognized, like:

.org
.dw
.bank

Are these considered ASM or NES-specific commands?

As far as I can tell they're quite common in 6502 assemblers. In the one I use (ca65), .dw is called .addr (sticks an address directly into the ROM), and .org doesn't even exist at all. (normally says "this code starts at this address" )

Posted by RetroRain

And I never used the stack. Is it possible to get by without it? Probably because I think documentation on it doesn't answer my questions.

How many set of numbers can you place on a stack, or is it unlimited, because the stack is not a RAM address right? I've seen no indication that it is. All I know is that it allows you to place numbers on it temporarily so you can quickly use them again.

The stack's 256 bytes big, and it resides in RAM from $100 to $1ff.


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blackhole89
Posted on 03-08-11 04:53 AM Link | Quote | ID: 140052


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Anything prefixed with a dots is macros for the assembler; they don't get translated to an opcode and therefore aren't NES-specific.

Also, I did SNES (rather than NES), but I found I almost never had to look at manuals because it was mostly trivial to infer from the command names, format of the arguments and the way they were used what the commands were supposed to do.

You can't entirely get around using the stack, simply because there are some special registers you can only access through it. Either way, it's a trivial data structure that every self-respecting programmer should understand; read up the wikipedia article.

Data on the stack is stored in the RAM. There is an address associated with the stack in the processor, the stack pointer, and the whole thing basically works like 
push x = decrement stack pointer by size of x and write x to the address pointed at by the stack pointer
pop(pull) x = set x to the data at the stack pointer and increment stack pointer by size of x

with the stack pointer initially pointing at something like $0200 for the NES (I think); note that it grows down. (Note: I might have the order of decrementing/incrementing the wrong way around; it depends on the system.)

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NovaYoshi
Posted on 03-08-11 05:19 AM Link | Quote | ID: 140053


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Posted by blackhole89

You can't entirely get around using the stack, simply because there are some special registers you can only access through it.


Special registers? wut
You'll need the stack for subroutine calls and interrupts. It's not really THAT scary... ( it scares me in x86 more )

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RetroRain
Posted on 03-08-11 05:32 AM Link | Quote | ID: 140054


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Posted by blackhole89
Also, I did SNES (rather than NES), but I found I almost never had to look at manuals because it was mostly trivial to infer from the command names, format of the arguments and the way they were used what the commands were supposed to do.
I'm sorry, I should have been more specific. I meant registers, like $4014 (Sprite DMA Register), $2000 (PPU Control Register 1), etc. Wouldn't it be pretty hard to remember every register for a specific system without having to look it up? I was just wondering if experienced NES programmers had these memorized by heart, and didn't have to refer to a document?

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Matrixz
Posted on 03-08-11 06:16 AM Link | Quote | ID: 140055


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Last view: 1776 days 		When i started to look at NES asm, i looked up the documentation all of the time, especially to know the details about those registers which contain bits, like $2000 and $2001.
Don't worry about it.

also, stack example: 
	lda #$01
	pha	;push
	lda #$02
	sta somewhere
	pla	;pull
		;Accumulator is 01

kuja killer
Posted on 03-08-11 07:21 AM Link | Quote | ID: 140056


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Last view: 42 min. 		yea for me, i look at documents when i need to, specifically when i dont remember what something does. Cause you cant just memorize everything obviously. Goes for all my megaman 3 stuff. I got 2 main notepad documents for megaman-odyssey i need open whenever im doing coding on stuff, so i can remember what commands and jumps and stuff to do for...whatever. Sorry if im not making much sense.

everything i do for odyssey is 100% pure custom in terms of programming for NES.

blackhole89
Posted on 03-08-11 02:37 PM (rev. 2 of 03-08-11 02:37 PM) Link | Quote | ID: 140061


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Posted by NovaYoshi
Special registers? wut

Ah, the NES only really has one of those (the processor flags are only directly accessible through by e.g. PHA, PLP to write and PHP, PLA to read). The SNES does the same thing for a couple more.

also,
> 04:11:20 <NovaYoshi> blackhole89 got the stack address wrong
No, note that the semantics for the stack pointer I've written down assume that it moves first, then writes; i.e. if it starts at $0200 for an empty stack (or $00 if SP is only 8-bit), the first byte to be pushed would occupy $01FF ($FF). This assumption may be wrong, but I made that explicit and if it was the other way around, the stack pointer indeed should start at $01FF.

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Ailure
Posted on 03-08-11 03:04 PM (rev. 3 of 03-08-11 03:09 PM) Link | Quote | ID: 140062

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As long you remember to have a equal numbers of push and pull in your code, the stack shouldn't really be that scary to use. If you manage to run out of stack space, that's probably a sign that your code is nested too deep anyway. I can't personally think of any examples of where that can easily happen on NES, but then most of the coding I done on the NES is rather simple.

For the most part you can get away with generic 6502 documentation, especially when it comes to opcodes and such (and I generally find it better than NES-spefic documentation). I used NES spefic documentation for the location of registers and such, and generic 6502 for opcodes (the biggest difference between a generic 6502 and the NES 2A03 opcode-wise is the lack of decimal mode as far I saw. Someone more experienced probably can point out way more differences.).

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never-obsolete
Posted on 03-08-11 11:15 PM Link | Quote | ID: 140073


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Last view: 2589 days 		After awhile you'll have it memorized. Especially for the NES, which only has a handful of registers compared to some of the other systems. I mostly need to reference cycle counts nowadays, but in the beginning I couldn't go more than 20 minutes without having to page through a document.

Main - ROM Hacking - If you've ever programmed the NES... New thread | New reply

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