Machine Code

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Overview

Machine Code is the base code of the 6502 processor at the heart of the VIC20. It is composed purely of 8bit numbers which the processor reads and then executes according to the current state of the processor. Many VIC20 programs are written in Machine Code, the VICs BASIC interpreter and Kernal are Machine Code programs. Unlike BASIC machine code is very fast. To run a BASIC program the VIC20 must read and interpret each BASIC token. Which may take many Machine Code instructions.

The same 5 Byte machine code program written in Hexadecimal, Binary and Decimal

Hex
A9 08 8D OF 90

Binary
10101001 00001000 10001101 00001111 10010000

Decimal
169, 8, 141, 15, 144

The rest of this article mainly uses Hexadecimal numbers, these are prefixed by the $ sign. So $80 is hex 80.

Assembly is just human readable machine code. It is converted into numbers by either a program or a human.

Assembly that the above machine code was created from
lda #$08
sta $900F

In order to convert from Assembly to Machine code one needs to know the opcodes for the instructions used. $A9 is the code for load accumulator immediate.

Registers

Addressing modes

Opcode listing

See this page for a full listing.

Mnemonic Hex Addressing type Instruction length Execution time
ADC #$44 $69 Immediate 2 bytes 2 cycles
ADC $44 $65 Zero Page 2 bytes 3 cycles
ADC $44,X $75 Zero Page,X 2 bytes 4 cycles
ADC $4400 $6D Absolute 3 bytes 4 cycles
ADC $4400,X $7D Absolute,X 3 bytes 4+ cycles
ADC $4400,Y $79 Absolute,Y 3 bytes 4 cycles
ADC ($44,X) $61 Indirect,X 2 bytes 6 cycles
ADC ($44),Y $71 Indirect,Y 2 bytes 5+ cycles

Branching

Arithmetic

Bit operations

Interrupts

Examples

Further Reading