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I have been trying to format binary opcodes for Motorola 68000, but I keep finding that it's not possible to encode both the destination memory address, instruction designation and addressing mode/size, and data value to be copied to the address bus for memory-mapped I/O.

For the Sega Genesis' Video Display Processor I am attempting to write to the control port which is memory mapped at C00004 in the Genesis' memory map.

C0004 is 1100 0000 0000 0000 0000 0100 in binary, or three bytes. The value I'm writing is 87, which the VDP recognizes as 8787 in VDP register #7. The issue I'm having is figuring out how to encode 32-bits worth of data, e.g. the instruction prefix designation move.b, value 87, which is #$87, and the destination memory address C00004 for MMIO re-routing to the correct VDP port on the way to the VDP.

Altogether it looks like this:

move.b  #$87, $00C00004,

which loosely translates into not four, but four bytes and a nibble(36-bits to be exact!)

0001 1000 0111 1100 0000 0000 0000 0000 0100

Since Motorola 68000 will only parse 32-bits when working down to microcode, how is it possible to encode the required information if there's not enough space(and within the same instruction)?

Perhaps I'm understanding this incorrectly?

I know this is beyond the level most programmers would anticipate, but I'm hoping someone around can break this down for me and explain how this encoding scheme would work.

Baby Wonder
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1 Answers1

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Your instruction, move.b #$87, $$00C00004 ought to encode to

0001111001111100 0000000010000111 00000000110000000000000000000100

(or similar; I'm not sure about the order of the operands).

The first 16-bit word can be broken down thusly:

  • The first four bits say that this is a move.b instruction.
  • The next six bits say that the destination addressing mode is an absolute 32-bit address.
  • The last six bits say that the source operand is immediate.

After that follows instruction extension words with the operands. The first is 16 bits for the immediate data, and the last 32 bits are for the address. (Might be the other way around.)

For more information, see http://www.freescale.com/files/archives/doc/ref_manual/M68000PRM.pdf

Lars Brinkhoff
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