CISS-111 Group Project 1
Write a disassembler for Commodore 64 (C64) program in the 6502/6510 assembly language.
Learning outcomes
- Working with a translation table.
- Working with existing data to build a new representation.
- Working with data type manipulations.
- Working with
switchexpressions. - Working with unsigned byte data.
- Working with unique file formats.
- Confirmation program produces desired results.
Historical Note
The VIC-20 only had 3,583 bytes available to programs once the system was up and running. By comparison, the Commodore 64 had 38,911 bytes.
Also, the 6502/6510 processors used were little-endian.
Setup
The file format for a C64 program is relatively simple:
Byte 0 - 1 -> load address in little-endian format. Byte 2 - end -> program as assembled from source.
Begin by opening the file passed as an argument. Read the first two bytes to determine where in memory this program is being loaded. Beginning at the next byte, set a program counter and begin disassembling instructions.
For each instruction you will need the following table:
import java.io.FileInputStream;
import java.io.IOException;
public class Dis6502 {
static Object[][] Opcodes = {
/* Name, Imm, ZP, ZPX, ZPY, ABS, ABSX, ABSY, IND, INDX, INDY, IMPL, REL */
{"ADC", 0x69, 0x65, 0x75, null, 0x6d, 0x7d, 0x79, null, 0x61, 0x71, null, null},
{"AND", 0x29, 0x25, 0x35, null, 0x2d, 0x3d, 0x39, null, 0x21, 0x31, null, null},
{"ASL", null, 0x06, 0x16, null, 0x0e, 0x1e, null, null, null, null, 0x0a, null},
{"BIT", null, 0x24, null, null, 0x2c, null, null, null, null, null, null, null},
{"BPL", null, null, null, null, null, null, null, null, null, null, null, 0x10},
{"BMI", null, null, null, null, null, null, null, null, null, null, null, 0x30},
{"BVC", null, null, null, null, null, null, null, null, null, null, null, 0x50},
{"BVS", null, null, null, null, null, null, null, null, null, null, null, 0x70},
{"BCC", null, null, null, null, null, null, null, null, null, null, null, 0x90},
{"BCS", null, null, null, null, null, null, null, null, null, null, null, 0xb0},
{"BNE", null, null, null, null, null, null, null, null, null, null, null, 0xd0},
{"BEQ", null, null, null, null, null, null, null, null, null, null, null, 0xf0},
{"BRK", null, null, null, null, null, null, null, null, null, null, 0x00, null},
{"CMP", 0xc9, 0xc5, 0xd5, null, 0xcd, 0xdd, 0xd9, null, 0xc1, 0xd1, null, null},
{"CPX", 0xe0, 0xe4, null, null, 0xec, null, null, null, null, null, null, null},
{"CPY", 0xc0, 0xc4, null, null, 0xcc, null, null, null, null, null, null, null},
{"DEC", null, 0xc6, 0xd6, null, 0xce, 0xde, null, null, null, null, null, null},
{"EOR", 0x49, 0x45, 0x55, null, 0x4d, 0x5d, 0x59, null, 0x41, 0x51, null, null},
{"CLC", null, null, null, null, null, null, null, null, null, null, 0x18, null},
{"SEC", null, null, null, null, null, null, null, null, null, null, 0x38, null},
{"CLI", null, null, null, null, null, null, null, null, null, null, 0x58, null},
{"SEI", null, null, null, null, null, null, null, null, null, null, 0x78, null},
{"CLV", null, null, null, null, null, null, null, null, null, null, 0xb8, null},
{"CLD", null, null, null, null, null, null, null, null, null, null, 0xd8, null},
{"SED", null, null, null, null, null, null, null, null, null, null, 0xf8, null},
{"INC", null, 0xe6, 0xf6, null, 0xee, 0xfe, null, null, null, null, null, null},
{"JMP", null, null, null, null, 0x4c, null, null, 0x6c, null, null, null, null},
{"JSR", null, null, null, null, 0x20, null, null, null, null, null, null, null},
{"LDA", 0xa9, 0xa5, 0xb5, null, 0xad, 0xbd, 0xb9, null, 0xa1, 0xb1, null, null},
{"LDX", 0xa2, 0xa6, null, 0xb6, 0xae, null, 0xbe, null, null, null, null, null},
{"LDY", 0xa0, 0xa4, 0xb4, null, 0xac, 0xbc, null, null, null, null, null, null},
{"LSR", null, 0x46, 0x56, null, 0x4e, 0x5e, null, null, null, null, 0x4a, null},
{"NOP", null, null, null, null, null, null, null, null, null, null, 0xea, null},
{"ORA", 0x09, 0x05, 0x15, null, 0x0d, 0x1d, 0x19, null, 0x01, 0x11, null, null},
{"TAX", null, null, null, null, null, null, null, null, null, null, 0xaa, null},
{"TXA", null, null, null, null, null, null, null, null, null, null, 0x8a, null},
{"DEX", null, null, null, null, null, null, null, null, null, null, 0xca, null},
{"INX", null, null, null, null, null, null, null, null, null, null, 0xe8, null},
{"TAY", null, null, null, null, null, null, null, null, null, null, 0xa8, null},
{"TYA", null, null, null, null, null, null, null, null, null, null, 0x98, null},
{"DEY", null, null, null, null, null, null, null, null, null, null, 0x88, null},
{"INY", null, null, null, null, null, null, null, null, null, null, 0xc8, null},
{"ROR", null, 0x66, 0x76, null, 0x6e, 0x7e, null, null, null, null, 0x6a, null},
{"ROL", null, 0x26, 0x36, null, 0x2e, 0x3e, null, null, null, null, 0x2a, null},
{"RTI", null, null, null, null, null, null, null, null, null, null, 0x40, null},
{"RTS", null, null, null, null, null, null, null, null, null, null, 0x60, null},
{"SBC", 0xe9, 0xe5, 0xf5, null, 0xed, 0xfd, 0xf9, null, 0xe1, 0xf1, null, null},
{"STA", null, 0x85, 0x95, null, 0x8d, 0x9d, 0x99, null, 0x81, 0x91, null, null},
{"TXS", null, null, null, null, null, null, null, null, null, null, 0x9a, null},
{"TSX", null, null, null, null, null, null, null, null, null, null, 0xba, null},
{"PHA", null, null, null, null, null, null, null, null, null, null, 0x48, null},
{"PLA", null, null, null, null, null, null, null, null, null, null, 0x68, null},
{"PHP", null, null, null, null, null, null, null, null, null, null, 0x08, null},
{"PLP", null, null, null, null, null, null, null, null, null, null, 0x28, null},
{"STX", null, 0x86, null, 0x96, 0x8e, null, null, null, null, null, null, null},
{"STY", null, 0x84, 0x94, null, 0x8c, null, null, null, null, null, null, null},
{"???", null, null, null, null, null, null, null, null, null, null, null, null}
};
// hexdump goes here...
private static void hexDump(byte[] bytes) {
//...
}
// disassemble goes here...
private static void disassemble (byte[] bytes) {
// ...
}
public static void main(String[] args) throws IOException {
FileInputStream in = new FileInputStream("congrats.prg");
byte[] file = in.readAllBytes();
in.close();
hexDump(file);
disassemble(file);
}
}
The table provides each form of opcode you may encounter. Note that the two-dimensional array is of type Object. This is because we are storing multiple data types and values necessary for the success and simplicity of the program. Even though the hexadecimal values are byte-sized, they will be stored as Integer and sign-extended. Be mindful of what this means.
The number of bytes and format of each addressing mode is in the table provided below.
Addressing Mode Example Bytes ---------- ------- ----- Immediate ORA #$44 2 Zero Page ORA $44 2 Zero Page,X LDA $44,X 2 Zero Page,Y STX $44,Y 2 Absolute ORA $4400 3 Absolute,X ORA $4400,X 3 Absolute,Y ORA $4400,Y 3 Indirect JMP $4400 3 Indirect,X ORA ($44,X) 2 Indirect,Y ORA ($44),Y 2 Implied INX 1 Relative BNE $4400 2
You will perform the following using this file as your confirmation:
- Translate opcode.
- Determine the addressing mode being used.
- Convert to full opcode with proper addressing form.
- Display each opcode with byte(s) and address.
Your output will be like the following:
3000: A9 25 85 FB A9 30 8D FC .%...0.. 3008: 00 A0 00 B1 FB F0 07 20 ....... 3010: D2 FF C8 4C 0B 30 A2 00 ...L.0.. 3018: BD 54 30 F0 07 20 D2 FF .T0.. .. 3020: E8 4C 18 30 60 20 20 20 .L.0` 3028: 20 D5 C4 C4 C4 C4 C4 C4 ....... 3030: C4 C4 C4 C4 C4 C4 C4 C4 ........ 3038: C4 C4 C9 0D 20 20 20 20 .... 3040: C7 43 4F 4E 47 52 41 54 .CONGRAT 3048: 55 4C 41 54 49 4F 4E 53 ULATIONS 3050: 21 C8 0D 00 20 20 20 20 !... 3058: C7 20 20 20 59 4F 55 20 . YOU 3060: 44 49 44 20 49 54 21 20 DID IT! 3068: 20 C8 0D 20 20 20 20 CA .. . 3070: C6 C6 C6 C6 C6 C6 C6 C6 ........ 3078: C6 C6 C6 C6 C6 C6 C6 C6 ........ 3080: CB 0D 00 ... 3000: A9 25 LDA #$25 3002: 85 FB STA $FB 3004: A9 30 LDA #$30 3006: 8D FC 00 STA $00FC 3009: A0 00 LDY #$00 300B: B1 FB LDA ($FB),Y 300D: F0 07 BEQ $3016 300F: 20 D2 FF JSR $FFD2 3012: C8 INY 3013: 4C 0B 30 JMP $300B 3016: A2 00 LDX #$00 3018: BD 54 30 LDA $3054,X 301B: F0 07 BEQ $3024 301D: 20 D2 FF JSR $FFD2 3020: E8 INX 3021: 4C 18 30 JMP $3018 3024: 60 RTS 3025: 20 20 20 JSR $2020 3028: 20 D5 C4 JSR $C4D5 302B: C4 C4 CPY $C4 302D: C4 C4 CPY $C4 302F: C4 C4 CPY $C4 3031: C4 C4 CPY $C4 3033: C4 C4 CPY $C4 3035: C4 C4 CPY $C4 3037: C4 C4 CPY $C4 3039: C4 C9 CPY $C9 303B: 0D 20 20 ORA $2020 303E: 20 20 C7 JSR $C720 3041: 43 ??? 3042: 4F ??? 3043: 4E 47 52 LSR $5247 3046: 41 54 EOR ($54,X) 3048: 55 4C EOR $4C,X 304A: 41 54 EOR ($54,X) 304C: 49 4F EOR #$4F 304E: 4E 53 21 LSR $2153 3051: C8 INY 3052: 0D 00 20 ORA $2000 3055: 20 20 20 JSR $2020 3058: C7 ??? 3059: 20 20 20 JSR $2020 305C: 59 4F 55 EOR $554F,Y 305F: 20 44 49 JSR $4944 3062: 44 ??? 3063: 20 49 54 JSR $5449 3066: 21 20 AND ($20,X) 3068: 20 C8 0D JSR $0DC8 306B: 20 20 20 JSR $2020 306E: 20 CA C6 JSR $C6CA 3071: C6 C6 DEC $C6 3073: C6 C6 DEC $C6 3075: C6 C6 DEC $C6 3077: C6 C6 DEC $C6 3079: C6 C6 DEC $C6 307B: C6 C6 DEC $C6 307D: C6 C6 DEC $C6 307F: C6 CB DEC $CB 3081: 0D ??? 3082: 00 ???
This seems like a complex project, but your contribution to the solution can be as little as 120 lines of code – with comments – if coded correctly. Development time can be reduced if you identify where cases can be combined and simplify the coding.
Your success in this project is based on the following criteria:
- A rough outline of the problem breakdown.
- Division of work between team members.
- Each member provides an equal contribution to the solution.
- A final one-page writeup of your solution with a self-assessment of what you think you did well and where you could improve the code.
Submit as Team#_Project1.java to the Learning Management System.