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luavirtual-machineopcodes

What's the difference between normal opcodes and opcodes with their MSB set?

What's the difference between normal opcodes and opcodes with their MSB (Most Significant Bit) set?

Example:

0036  5E000001           [4] return     1   2
003A  1E008000           [5] return     0   1

The first opcode (0x5E/1011110) has its MSB set and the second opcode (0x1E/0011110) hasn't.

Edit: Corrected 'byte' to 'bit', duh.

Solution

  • After some googling, I found a header of the Lua source (lopcodes.h) explaining the instruction format a bit better.

    It looks like it's wrong to read the opcode as a byte, it should be read as 6 bits:

    All instructions have an opcode in the first 6 bits.
Instructions can have the following fields:
`A' : 8 bits
`B' : 9 bits
`C' : 9 bits
`Bx' : 18 bits (`B' and `C' together)
`sBx' : signed Bx