Creating a bootrom.

[jackliddle]

Hi

Im putting together a 6502 based computer on an FPGA. I want to put a 4k ROM starting at 0xF000h.

What I don't understand is how to create this ROM in assembler. I have been using the xa65 assembler. The problems I am unable to solve are.

1) Getting the assembler to output a file exactly 4K long so I can convert it too VHDL easily.

2) Placing bytes at specific positions in this ROM (e.g. at byte FFFC to determine on reset behaviour).

3) Having all code labels from 0xF000h -> 0xFFFFh.

Hopefully my problems makes sense. I have poured over the man pages but am none the wiser. If anyone know how to solve these problems, or where I might go too look for a solution I would be very grateful.

Thanks

Jack

[GARTHWILSON]

1. The unused space is "don't care." IOW, you can fill it with FF's or leave it random, or whatever. The file doesn't have to be for exactly 4K; but if you want it to be, use whatever kind of FILL directive your assembler has to fill the space you don't use.

2. The assembler directive on the assemblers I've used (and it's probably almost universal) to tell it where to start laying down code is ".org" for "origin" of the following code. So the line before the vectors will say something like:

[fachat]

Since I am the original author of xa65, I can probably help...

[kc5tja]

[NattyFido]

[BigDumbDinosaur]

[GARTHWILSON]

[kc5tja]

But Garth, in every single case you listed, it's 100% unambiguous what you mean. 0xF00 is likewise unambiguous when you're talking about software written in C. You will never see 'x' without that leading zero.

For the same reason, is BEEFH a label? Or, is it a hexadecimal constant? It could well be both. What determines which is that leading zero, again. 0BEEFH is the constant, BEEFH is the label.

You just have to know what context you're talking in.

[BigDumbDinosaur]

[kc5tja]

Wow -- and you call ME confrontational...


I have a better explanation.

C has, in effect, dominated 99% of the software development conducted on any arbitrary microprocessor you care to name, either directly (it's displaced Forth as the language du jour for embedded development) or indirectly. It's literally everywhere -- it features itself quite prominently in languages like C++, C#, Java, and perhaps to a lesser degree, Perl. So, given the statistical probability of prior C exposure by the author, it is natural that he used 0x-notation without even thinking about it. It's not an issue of it being "better". Further, you won't find evidence of it being superior or inferior because you cannot objectively measure subjective opinions, impacts on parser design in any meaningful way, or even ease of IDE implementation. So, it's not even worth it to try. 0x-notation can be parsed with LL or LR grammars, and both parser- and lexer-generator tools make quick work of supporting 0x-notation without batting an eyebrow. Even if the resulting parser becomes somewhat more complex (perhaps a reduction rule or two at most), the notation's popularity ensures that's just one less thing you need to train new users of your product on, so there's just no question that that complexity will pay itself off on its own.

Additionally, an increasing number of assemblers now support this notation. It's the only system supported by the AVR 8-bit assemblers (I presume the same is true for their 32-bit line-up too) I've ever used. It's recognized, if not actively used, in every 80x86 assembler I've used, Unix or Windows alike. It's also dominated the hex notation used in virtually every RISC processor assembly syntax on the market today.

P.S.: I'm not sure if I mentioned this before, but the $-notation actually started with Motorola, as I indicated before, with the release of the 6800 microprocessor and its documentation. This is not a MOS Technologies original.

[kc5tja]

I'd like to point out that I prefer $-notation because it's more comfortable for me to type, not because I have an easier time reading it. I have no problems what-so-ever discerning 0x-notation hex constants from non-hex constants.

So, I'm not arguing in favor of 0x-notation. I am, however, trying to keep the discussion as objective as possible. And, I note that we've veered horrifically from the original topic of this thread.

[fachat]

[DaveK]

I'm using the ASxxxx collection of assemblers, because it covers most of the processors I want using similar syntax, and is relocatable. It requires 0x even on the 6502.