Subject: Re: [stella] Hello from Square One From: Robin Harbron <macbeth@xxxxxxxxxxx> Date: Wed, 06 Jan 1999 19:15:24 -0500 |
Pete Holland wrote: > 1) I was looking over some of the disassembled code printouts and I > notice that there are no "memory locations" (roughly analogous to line > numbers in BASIC?) listed by the code. So how is the code put in? > Where to the memory location assignments come in? And if you just > start typing everything in, how do you know where the JSR and JMP > commands are supposed to lead to? That's the beauty of an assembler - you can avoid having to know every last detail of the program you're working on - most of the time, anyway. Whenever you want to refer to a particular point in a program, you just use a label, like: here inx jsr there jmp here there rts I hope that's self-explanatory. The assembler worries about what actual memory location "here" and "there" refer to. Most assemblers use a command like: ORG $1000 or *= $1000 to tell the assembler the starting address to assemble to. > 2) I was reading a book on programming the Apple IIe (6502 processor, > doncha know), and it mentions that the branch commands only go 127 (or > 128, depending on direction) bytes at the most. Has anybody ever run > into a situation where branches are required further than this? Most of the time it's not a problem. If you require a larger branch, you can just negate the branch instruction, and add a JMP, eg: loop inx bne loop Will become: loop inx beq ahead jmp loop ahead > 4) Supposing I JSR to a JMP command. Will a RTS send the program > back to the command after JSR, or will I screw up the program? As long as you don't mess up the stack yourself, it'll work fine. > 5) When the Atari powers up, are all the memory locations, flags, > etc. reset automatically, or in some random state so the programmer > needs to reset them himself? It's best to reset it yourself - never assume anything. > 6) Is there a way to produce text for a few words on the Atari, or do > any words have to be drawn out by the graphics engine (is that the > right term?)? Everything has to be drawn scan line by scan line. > images (if you have the memory, of course). But why bother? The game > is better suited to graphics engines that can display the curiosity > and wonder of the images. Even if the Atari had 1 gigabyte, it > wouldn't work, if you will. So I move that we leave the Atari and its > limitations alone, and deal with them. But there are plenty of nifty things the Atari can do when it's given a bit of extra RAM - just check out a bunch of the great games that have been made in the bigger formats. Those games never would have happened in only 4k - and don't try to tell me that they're not classic 2600 games... -- Robin Harbron macbeth@xxxxxxxxxxx http://www.tbaytel.net/macbeth -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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