Atari 2600 Programming for Newbies

Symbol Table

Whenever DASM is doing its job assembling, it keeps a list of all the 'words' it encounters in a file in an internal structure called a symbol table. Think of a symbol as a name for something. Remember the 'sta WSYNC' instruction we used to halt the 6502 and wait for the scanline to be rendered? The 'sta' is the instruction, and 'WSYNC' is a symbol. When it first encounters this symbol, DASM doesn't know much about it, other than what it's called (ie: 'WSYNC'). What DASM needs to do is work out what the *value* of that symbol is, so that it can insert that value into the binary file.

When it's assembling, DASM puts all the symbols it finds into its symbol table—and associated with each of these is a value. If it doesn't 'know' the value, that's OK—DASM will keep assembling the rest of the file quite happily. At some point, something in the code might tell DASM what the value for a symbol actually IS—in which case DASM will put that value in its symbol table alongside the symbol. So whenever that symbol is used anywhere, DASM now knows its correct value to put into the binary file.

In fact, it is absolutely necessary for all symbols which go into the binary file to be given values at some point. DASM can't guess values—it's up to you, the programmer, to make sure this happens. A symbol doesn't have to be given a value at any PARTICULAR point in the code, but it does have to be given a value somewhere in the code. DASM will make multiple 'passes'—basically going through the code from beginning to end again and again until it manages to resolve all the symbols to correct values.

     include "vcs.h"

So now we know basically what DASM does with symbols—it keeps an internal list of symbols—and their values, if known. DASM will keep going through the code and 'resolving' the symbols into numeric values, until it is complete (or it couldn't find ANYTHING to resolve, in which case it gives an error). Once all symbols have been resolved, your code has been completely processed by the assembler, and it creates the binary image/file for you—and assembly is complete.

DASM Summary

To summarize: DASM converts source-code consisting of instructions (mnemonics) and symbols into a binary form which can be run by the 6502. The assembler converts mnemonics into opcodes (numbers), and symbols into numbers which it calculates the value of during the assembly process.

Command Line

DASM is a command-line program—that is, it runs under DOS (or whatever platform you happen to choose, provided you have a runnable version for that platform). DASM is provided with full source-code (it's written in C) so as long as you have a C-compiler handy, you can port it to just about any platform under the sun.

It does come with a manual—and it's always a good idea to familiarize yourself with its capabilities. In the interests of getting you up and running quickly, so you can actually assemble the sample kernel posted a session or two ago, here's what you need to type on the command-line…

 dasm kernel.asm -lkernel.txt -f3 -v5 -okernel.bin

This is assuming that the file to assemble is named 'kernel.asm' (.asm is a standard prefix for assembler files, but some prefer to use .s—you can use whatever you want, really, but I always use .asm). Anything prefixed with a minus-sign ('-') is a 'switch'—which tells DASM something about what it is required to do. The -l switch we discussed very briefly, and that tells DASM to create a listing file—in this case, it will write a listing to the file 'kernel.txt'. The -o switch tells DASM what file to use for the output binary—in this case, the binary will be written to 'kernel.bin'. That file can be loaded into an emulator, or burned on an EPROM—it is the ROM file, in other words.

The other switches '-f3' and '-v5' control some internals of DASM—and for now just assume you need these whenever you assemble with DASM. Remember, if you're curious you can always read the manual!

Output

If all goes well, DASM will output something like this…

DASM V2.20.05, Macro Assembler (C)1988-2003

START OF PASS: 1

----------------------------------------------------------------------

SEGMENT NAME                 INIT PC  INIT RPC FINAL PC FINAL RPC

                             f000                            f000

RIOT                     [u] 0280                            0280

TIA_REGISTERS_READ       [u] 0000                            0000

TIA_REGISTERS_WRITE      [u] 0000                            0000

INITIAL CODE SEGMENT         0000 ????                       0000 ????

----------------------------------------------------------------------

1 references to unknown symbols.

0 events requiring another assembler pass.

--- Symbol List (sorted by symbol)

AUDC0                    0015

AUDC1                    0016

AUDF0                    0017

AUDF1                    0018

AUDV0                    0019

AUDV1                    001a

COLUBK                   0009              (R )

COLUP0                   0006

COLUP1                   0007

COLUPF                   0008

CTRLPF                   000a

CXBLPF                   0006

CXCLR                    002c

CXM0FB                   0004

CXM0P                    0000

CXM1FB                   0005

CXM1P                    0001

CXP0FB                   0002

CXP1FB                   0003

CXPPMM                   0007

ENABL                    001f

ENAM0                    001d

ENAM1                    001e

GRP0                     001b

GRP1                     001c

HMBL                     0024

HMCLR                    002b

HMM0                     0022

HMM1                     0023

HMOVE                    002a

HMP0                     0020

HMP1                     0021

INPT0                    0008

INPT1                    0009

INPT2                    000a

INPT3                    000b

INPT4                    000c

INPT5                    000d

INTIM                    0284

NUSIZ0                   0004

NUSIZ1                   0005

Overscan                 f02c              (R )

PF0                      000d

PF1                      000e

PF2                      000f

Picture                  f01d              (R )

REFP0                    000b

REFP1                    000c

RESBL                    0014

Reset                    f000              (R )

RESM0                    0012

RESM1                    0013

RESMP0                   0028

RESMP1                   0029

RESP0                    0010

RESP1                    0011

RSYNC                    0003

StartOfFrame             f000              (R )

SWACNT                   0281

SWBCNT                   0283

SWCHA                    0280

SWCHB                    0282

T1024T                   0297

TIA_BASE_ADDRESS         0000              (R )

TIM1T                    0294

TIM64T                   0296

TIM8T                    0295

TIMINT                   0285

VBLANK                   0001              (R )

VDELBL                   0027

VDELP0                   0025

VDELP1                   0026

VerticalBlank            f014              (R )

VSYNC                    0000              (R )

WSYNC                    0002              (R )

--- End of Symbol List.

Complete.