Difference between revisions of "Motorola 68000"

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=== The Motorola 68k processor  ===
 
=== The Motorola 68k processor  ===
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The Motorola 68k processor...  
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The Motorola 68k processor is a 16-bit Big-Endian format processor, using 32-bit register and addresses.
  
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Note: Assigment direction is source,dest instead of dest,source !!!
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This means that instruction sizes on average are either 2 or 4 bytes each, and 6 bytes for longword / 32-bit instructions.
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Note: The assigment direction is source,dest instead of dest,source !!!
  
 
==== Registers ====
 
==== Registers ====
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To be added.
 
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== Atari ST ==
 
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The Atari ST systems consists of the M68k system with custom hardware for graphics and sound.
 
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=== Setting up ===
 
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Setting up your development platform for the Atari ST systems is quite easy, first get the following tools:
 
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* Assembler: VASM - This assembler is able to assemble directly to a TOS image
 
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* Emulator(s): -. Make sure to use the original TOS 1.62 Image for best compatibility.
 
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==== Compiling to a TOS image ====
 
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Vasm -Ftos source.s -o source.tos
 
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=== Video display ===
 
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The Atari ST uses an interleaved planar memory layout to represent it's paletted display modes (we'll concentrate on 320x200x16 colours here).
 
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==== Paletted? ====
 
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The Atari ST uses index values into a palette of colours. Index 0 is the background colour (that's also used for the border) and a maximum of 16 colours can be defined and indexed.
 
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==== Planes? ====
 
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Every plane contains one bit of a pixel's colour index value. The bits of the binary represantation of a colour index like %1010 (% Bit3,Bit2,Bit1,Bit0) will end up in 4 different planes (bits most significant to least significant aka left to right): Plane4 Plane3 Plane2 Plane1.
 
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So basicly Plane1 contains all of the Bit0s of all pixels, Plane2 all Bit1s, Plane3 all Bit2s and Plabe4 all Bit3s.
 
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The first pixel on a plane is described by the leftmost (aka most significant) bit in a word, the second one by the second-leftmost etc. - just like this %0123456789abcdef with 0-f=pixels 1-16. %1000000000000000=$8000=pixel 1 in a plane word set.
 
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==== Interleaved? ====
 
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16 pixels worth of data are represented as a full graphicword, meaning all information to display 16 pixels are stored together, followed by the data to represent the next 16 pixels etc. One row worth of display data has 20 graphicwords (20*16 pixels=320 pixels).
 
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16 pixels are stored in 4 words - which contain 4 of the aforementioned planes.
 
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<br /><br />
 
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So a 320x200x16 colour display is a contiuous memory buffer containing:
 
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<syntaxhighlight lang="">
 
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Pixels 0-15, row 0:(Plane1.w Plane2.w Plane3.w Plane4.w)
 
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Pixels 16-31, row 0:(Plane1.w Plane2.w Plane3.w Plane4.w)
 
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Pixels 32-47, row 0:(Plane1.w Plane2.w Plane3.w Plane4.w)
 
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......
 
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Pixels 304-319, row 199:(Plane1.w Plane2.w Plane3.w Plane4.w)
 
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</syntaxhighlight>
 
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To be refined soon.
 
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==== Setting a palette ====
 
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Here is some code that will help you setup a palette
 
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<syntaxhighlight lang="">
 
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pea palette(pc)
 
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move.w #6,-(sp)
 
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trap #14
 
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; Palette data
 
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palette:
 
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dc.w $000,$100,$200,$311,$422,$533,$644,$755
 
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dc.w $575,$464,$353,$242,$131,$020,$010,$000
 
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</syntaxhighlight>
 
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==== Getting something on screen ====
 
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Here is a bit of code to get you started:
 
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<syntaxhighlight lang="">
 
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;-----------------------
 
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; Line-A Initialization
 
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;-----------------------
 
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; After calling this function, data register D0 and address register A0 point to a table ; with the starting address of the Line A variables.
 
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; Address register A1 points to a table with the starting addresses for the three system ; font headers,
 
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; and address register A2 points to a table that specifies the starting addresses of the; 15 Line A opcodes. There's no parameter required for this function, so all you have
 
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; to do is call the word opcode label that you specified for the $A000 (Initialize)
 
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; function.
 
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dc.w $A000
 
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movem.l (a0),a1-a4 ; A3=INTIN, A4=PTSIN
 
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;---------
 
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; For X&Y
 
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;---------
 
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frameloop:
 
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move.w #200-1,d7 ; y
 
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yLoop:
 
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move.w #320-1,d6 ; x
 
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xLoop:
 
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; Putpixel
 
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put_pixel:
 
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move.b d6,d0 ; d0=x
 
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eor d7,d0 ; d0=x^y
 
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lsr.b #2,d0 ; d0>>=4
 
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and #42,d0 ; d0&42
 
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move.w d0,(a3) ; a3=color(d0)
 
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movem.w d6/d7,(a4) ; a4=x,y`
 
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dc.w $A001 ; put pixel command
 
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dbra d6,xLoop ; decrease and branch
 
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    dbra d7,yLoop
 
  
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; Wait loop
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* D0..D7 - 8 x 32 bit General Purpose Registers
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bra frameloop ; .s *
 
  
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</syntaxhighlight>
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* A0..A6 - 7 x 32 bit Address Registers
  
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=== Sound ===
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* A7 - 32-bit Stack-Address Register
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The Atari ST systems use the YM2149 chip to generate sound.\
 
  
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For more information check out https://www.atarimagazines.com/v4n7/stsound.html
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==== Instructions timing ====
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The number of cycles for each instruction is different depending of processor model in M68K family.
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: http://oldwww.nvg.ntnu.no/amiga/MC680x0_Sections/mc68000timing.HTML
  
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==== Make some noise ====
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== Size considerations ==
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To be added soon.
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Here are some general rule of thumbs when it comes to size consideration when programming the M68000
  
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=== Additional Resources ===
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* Moving/Calculating Register from/to registers - 2 bytes
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Sizecoding on the Atari ST is not very huge yet, so resources are sparse.
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* Moving/Calculating with byte or word values - 4 bytes
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* ST Soundchip: https://www.atarimagazines.com/v4n7/stsound.html
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* Moving/Calculating with long values - 6 bytes
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* Spkr's Github: to be added
 
  
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== Commodore Amiga ==
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Shorter variants:
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The Commodore Amiga system consists of the M68k system with custom hardware for graphics and sound.
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* moveq #value, reg : 2 bytes - Moves a values -128...127 to a register
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* addq #value, reg : 2 bytes - Adds a values 0..8 to a register
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* subq #value, reg : 2 bytes - Subtracts a values 0..8 from a register
  
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=== Setting up ===
 
  
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* Assembler: -
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== Generic 68K sinus table generator ==
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* Emulator(s): WinUAE
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Here is a fairly generic 68k sinus table generator (16 bytes on Atari ST) as well as a 10-byte zigzag generator.
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* [https://demozoo.org/productions/310191/ Singen68k]
  
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=== Video display ===
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== Resources ==
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No information yet
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* [http://www.beycan.net/eklenen/M68000_Instruction_Set.pdf M68000 Instruction Set]
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* [http://www.easy68k.com/ EASy68K Editor/Assembler/Simulator for the 68000]
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* [http://www.easy68k.com/paulrsm/index.html Various Motorola 68k resources]
  
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=== Sound ===
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== Motorola M68K Platforms ==
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No information yet
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*'''[[Atari ST]]''' - Atari ST Sizecoding information
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*'''[[Atari Jaguar]]''' - Atari Jaguar Sizecoding information
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*'''[[Commodore Amiga]]''' - Commodore Amiga Sizecoding information
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*'''[[Spectrum QL]]''' - Spectrum QL information

Latest revision as of 12:51, 8 April 2024

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


Generic 68K sinus table generator

Here is a fairly generic 68k sinus table generator (16 bytes on Atari ST) as well as a 10-byte zigzag generator.

Resources

Motorola M68K Platforms

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