How do 68,000 internally present instructions?

Question

How 68,000 internally represent instructions. I read that there are different types of instructions: one-format operational word format instructions, short and full word format instructions. It seems that one effective working word command is an instruction, and the lower 6 bits of this instruction are addressing mode and register. Does this addressing mode and register describe whether to follow short or complete instructions for expanding a word, which in its turn represents operands for an instruction. You know a better guide than the 68000 Programming Reference.

Thanks in advance

Answer 1

The actual internal representation is a combination of microcode and nanocode. The 68000 has 544 17-bit microcode words that transmit 366 68-bit nanocode words.

Although this may not be what you wanted to know, this link may provide some information:

http://www.easy68k.com/paulrsm/doc/dpbm68k1.htm

Answer 2

to the right, the m68000 indexed modes use a short extension. In the section “Address register with indirect index (8-bit offset mode)” (d8, An, Xn) BEW is filled in D / A (if Xn is a data or address register), Xn (register number), W / L (for threat Xn content as 16 or 32 bits), scale to 0 (see Note) and 8-bit offset.

on the other hand, in other modes, such as a 16-bit offset, “Address with an offset” (d16, An), the extension is just a word with an offset.

note: short extension word - m68k does not support 2 bits for scale, so it is set to 0; scale on BEW using scale bits, and full extensions are only supported by m68020,40, → cpus. http://etd.dtu.dk/thesis/264182/bac10_19.pdf