Motorola 68000
Contents
[hide]Introduction
Wanting to start sizecoding on a Motorola 68000 platforms in this day and age can be tough. So lets start with a list of well known Motorola 68000 systems you could target for sizecoding.
Motorola 68000 Platforms
Here is a list of well known Motorola 68000 platforms you could target for sizecoding:
- Atari ST - Atari ST Sizecoding information
- Commodore Amiga - Commodore Amiga Sizecoding information
- Atari Jaguar - Atari Jaguar Sizecoding information
- Spectrum QL - Spectrum QL information
- X68000 - Sharp X68000 information
The Motorola 68000 processor
The Motorola 68000/68k 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.
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
Generic 68K sinus table generator
Here is a fairly generic 68k sinus table generator (14 bytes on Atari ST/Amiga) as well as a 10-byte zigzag generator.
