Motorola 68000

Introduction

Wanting to start sizecoding on a Motorola 68k platform in this day and age can be tough.

So here is a bit of help to get you started:

The Motorola 68k processor

The Motorola 68k processor is a 16-bit Big-Endian format processor, using 32-bit register and addresses.

This means that instruction sizes on average are either 2 or 4 bytes each, and 6 bytes for longword / 32-bit instructions.

Note: The assigment direction is source,dest instead of dest,source !!!

Registers

• D0..D7 - 8 x 32 bit General Purpose Registers

• A0..A6 - 7 x 32 bit Address Registers

• A7 - 32-bit Stack-Address Register

Instructions timing

The number of cycles for each instruction is different depending of processor model in M68K family.

http://oldwww.nvg.ntnu.no/amiga/MC680x0_Sections/mc68000timing.HTML

Size considerations

Here are some general rule of thumbs when it comes to size consideration when programming the M68000

• Moving/Calculating Register from/to registers - 2 bytes
• Moving/Calculating with byte or word values - 4 bytes
• Moving/Calculating with long values - 6 bytes

Shorter variants:

• moveq #value, reg : 2 bytes - Moves a values -128...127 to a register
• addq #value, reg : 2 bytes - Adds a values 0..8 to a register
• subq #value, reg : 2 bytes - Subtracts a values 0..8 from a register

Resources

• M68000 Instruction Set
• EASy68K Editor/Assembler/Simulator for the 68000
• Various Motorola 68k resources

Motorola M68K Platforms

• Atari ST - Atari ST Sizecoding information
• Atari Jaguar - Atari Jaguar Sizecoding information
• Commodore Amiga - Commodore Amiga Sizecoding information