I was looking for a simple 6502 disassembler a couple days ago and couldn't really find what I wanted.
I found a great assortment of Windows based disassemblers, but being a Linux user, I generally keep away from Windows, and while I did find some disassembler code that was happy to compile, the result appeared to target a specific assembler. All's I wanted was a standard disassembly like the 'd' command in the old 6502 monitor programs. Perhaps I didn't look hard enough.
So yesterday I wrote one, MOS 6502 Instruction set only (though an update to 65c02 should be quite simple), and outputs standard disassembly like the old monitors.
Attached along with the source is a compiled version for Windows users (command line).
The source is shorter than some of the 6502 ASM code posted around here, so here it is in all its ugliness.
I really don't do much C coding (next to none) so there might be some silliness in the code that needs pointing out to me.
Code:
/* d6502 v0.4 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char **argv) {
FILE *file;
char *buffer;
unsigned long fileLen;
int address;
int i;
int currentbyte;
int previousbyte;
int paramcount;
int addrmode;
char *opcode;
char *pad;
char *pre;
char *post;
// Padding for 1,2 & 3 byte instructions
char *padding[3] = {" "," ",""};
// 57 Instructions + Undefined ("???")
char *instruction[58] = {
// 0 1 2 3 4 5 6 7 8 9
"ADC","AND","ASL","BCC","BCS","BEQ","BIT","BMI","BNE","BPL", // 0
"BRK","BVC","BVS","CLC","CLD","CLI","CLV","CMP","CPX","CPY", // 1
"DEC","DEX","DEY","EOR","INC","INX","INY","JMP","JSR","LDA", // 2
"LDX","LDY","LSR","NOP","ORA","PHA","PHP","PLA","PLP","ROL", // 3
"ROR","ROT","RTI","RTS","SBC","SEC","SED","SEI","STA","STX", // 4
"STY","TAX","TAY","TSX","TXA","TXS","TYA","???"}; // 5
// This is a lookup of the text formating required for mode output, plus one entry to distinguish relative mode
char *modes[9][2]={{"",""},{"#",""},{"",",X"},{"",",Y"},{"(",",X)"},{"(","),Y"},{"(",")"},{"A",""},{"",""}};
// Opcode Properties for 256 opcodes {length_in_bytes, mnemonic_lookup, mode_chars_lookup}
int opcode_props[256][3] = {
// 0 1 2 3 4 5 6 7 8 9 A B C D E F
// ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** --------
{1,10,0},{2,34,4},{1,57,0},{1,57,0},{1,57,0},{2,34,0},{2,2,0}, {1,57,0},{1,36,0},{2,34,1},{1,2,7}, {1,57,0},{1,57,0},{3,34,0},{3,2,0}, {1,57,0}, // 0
{2,9,8}, {2,34,5},{1,57,0},{1,57,0},{1,57,0},{2,34,2},{2,2,2}, {1,57,0},{1,13,0},{3,34,3},{1,57,0},{1,57,0},{1,57,0},{3,34,2},{3,2,2}, {1,57,0}, // 1
{3,28,0},{2,1,4}, {1,57,0},{1,57,0},{2,6,0}, {2,1,0}, {2,39,0},{1,57,0},{1,38,0},{2,1,1}, {1,39,7},{1,57,0},{3,6,0}, {3,1,0}, {3,39,0},{1,57,0}, // 2
{2,7,8}, {2,1,5}, {1,57,0},{1,57,0},{1,57,0},{2,1,2}, {2,39,2},{1,57,0},{1,45,0},{3,1,3}, {1,57,0},{1,57,0},{1,57,0},{3,1,2}, {3,39,2},{1,57,0}, // 3
{1,42,0},{2,23,4},{1,57,0},{1,57,0},{1,57,0},{2,23,0},{2,32,0},{1,57,0},{1,35,0},{2,23,1},{1,32,7},{1,57,0},{3,27,0},{3,23,0},{3,32,0},{1,57,0}, // 4
{2,11,8},{2,23,5},{1,57,0},{1,57,0},{1,57,0},{2,23,2},{2,32,2},{1,57,0},{1,15,0},{3,23,3},{1,57,0},{1,57,0},{1,57,0},{3,23,2},{3,32,2},{1,57,0}, // 5
{1,43,0},{2,0,4}, {1,57,0},{1,57,0},{1,57,0},{2,0,0}, {2,40,0},{1,57,0},{1,37,0},{2,0,1}, {1,40,7},{1,57,0},{3,27,6},{3,0,0}, {3,40,0},{1,57,0}, // 6
{2,12,8},{2,0,5}, {1,57,0},{1,57,0},{1,57,0},{2,0,2}, {2,40,2},{1,57,0},{1,47,0},{3,0,3}, {1,57,0},{1,57,0},{1,57,0},{3,0,2}, {3,40,2},{1,57,0}, // 7
{1,57,0},{2,48,4},{1,57,0},{1,57,0},{2,50,0},{2,48,0},{2,49,0},{1,57,0},{1,22,0},{1,57,0},{1,54,0},{1,57,0},{3,50,0},{3,48,0},{3,49,0},{1,57,0}, // 8
{2,3,8}, {2,48,5},{1,57,0},{1,57,0},{2,50,2},{2,48,2},{2,49,3},{1,57,0},{1,56,0},{3,48,3},{1,55,0},{1,57,0},{1,57,0},{3,48,2},{1,57,0},{1,57,0}, // 9
{2,31,1},{2,29,4},{2,30,1},{1,57,0},{2,31,0},{2,29,0},{2,30,0},{1,57,0},{1,52,0},{2,29,1},{1,51,0},{1,57,0},{3,31,0},{3,29,0},{3,30,0},{1,57,0}, // A
{2,4,8}, {2,29,5},{1,57,0},{1,57,0},{2,31,2},{2,29,2},{2,30,3},{1,57,0},{1,16,0},{3,29,3},{1,53,0},{1,57,0},{3,31,2},{3,29,2},{3,30,3},{1,57,0}, // B
{2,19,1},{2,17,4},{1,57,0},{1,57,0},{2,19,0},{2,17,0},{2,20,0},{1,57,0},{1,26,0},{2,17,1},{1,21,0},{1,57,0},{3,19,0},{3,17,0},{3,20,0},{1,57,0}, // C
{2,8,8}, {2,17,5},{1,57,0},{1,57,0},{1,57,0},{2,17,2},{2,20,2},{1,57,0},{1,14,0},{3,17,3},{1,57,0},{1,57,0},{1,57,0},{3,17,2},{3,20,2},{1,57,0}, // D
{2,18,1},{2,44,4},{1,57,0},{1,57,0},{2,18,0},{2,44,0},{2,24,0},{1,57,0},{1,25,0},{2,44,1},{1,33,0},{1,57,0},{3,18,0},{3,44,0},{3,24,0},{1,57,0}, // E
{2,5,8}, {2,44,5},{1,57,0},{1,57,0},{1,57,0},{2,44,2},{2,24,2},{1,57,0},{1,46,0},{3,44,3},{1,57,0},{1,57,0},{1,57,0},{3,44,2},{3,24,2},{1,57,0} // F
};
if (argc < 2) { //If no parameters given, display usage instructions and exit.
fprintf(stderr, "Usage: %s filename address\n\n", argv[0]);
fprintf(stderr, "Example: %s dump.rom E000\n", argv[0]);
exit(1);
}
if (argc == 3) {
address = strtol(argv[2], NULL, 16); //If second parameter, accept it as HEX address for start of dissasembly.
}
file = fopen(argv[1], "rb"); //Open file
if (!file) {
fprintf(stderr, "Can't open file %s", argv[1]); //Error if file not found
exit(1);
}
fseek(file, 0, SEEK_END); //Seek to end of file to find length
fileLen = ftell(file);
fseek(file, 0, SEEK_SET); //And back to the start
buffer = (char * ) malloc(fileLen + 1); //Set up file buffer
if (!buffer) { //If memory allocation error...
fprintf(stderr, "Memory allocation error!"); //...display message...
fclose(file); //...and close file
exit(1);
}
fread(buffer, fileLen, 1, file); //Read entire file into buffer and...
fclose(file); //...close file
paramcount = 0;
printf(" * = $%04X \n", address); //Display org address
for (i = 0; i < fileLen; ++i) { //Start proccessing loop.
previousbyte = currentbyte;
currentbyte = ((unsigned char * ) buffer)[i];
if (paramcount == 0) {
printf("$%04X ", address); //Display current address at beginning of line
paramcount = opcode_props[currentbyte][0]; //Get instruction length
opcode = instruction[opcode_props[currentbyte][1]]; //Get opcode name
addrmode = opcode_props[currentbyte][2]; //Get info required to display addressing mode
pre = modes[addrmode][0]; //Look up pre-operand formatting text
post = modes[addrmode][1]; //Look up post-operand formatting text
pad = padding[(paramcount - 1)]; //Calculate correct padding for output alignment
address = address + paramcount; //Increment address
}
if (paramcount != 0) //Keep track of possition within instruction
paramcount = paramcount - 1;
printf("$%02X ", currentbyte); //Display the current byte in HEX
if (paramcount == 0) {
printf(" %s %s %s", pad, opcode, pre); //Pad text, display instruction name and pre-operand chars
if(!strcmp (pad," " )) { //Check if single operand instruction
if (addrmode != 8) { //If not using relative addressing ...
printf("$%02X", currentbyte); //...display operand
} else { //Addressing mode is relative...
printf("$%04X", (address + ((currentbyte < 128) ? currentbyte : currentbyte - 256))); //...display relative address.
}
}
if(!strcmp (pad,"" )) //Check if two operand instruction and if so...
printf("$%02X%02X", currentbyte, previousbyte); //...display operand
printf("%s\n", post); //Display post-operand chars
}
}
printf("$%04X .END\n", address); //Add .END directive to end of output
free(buffer); //Return buffer memory to the system
return 0; //All done, exit to the OS
}
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