mruby/c VM executes the series of instructions, mruby/c bytecode.
Each bytecode contains op-code and operands. Op-code is always represented by one byte and all op-codes are shown in opcode.h.
Operands is variable bytes depends on op-code. The operands are also shown in opcode.h as following manner.
The operand column shows the combination of {Z,B,S,W}. Each means that the following operands appear in that order.
| operand | type of operand | length (bytes) |
|---|---|---|
| Z | no operand | 0 |
| B | 8 bits operand | 1 |
| S | 16 bites operand | 2 |
| W | 24 bits operand | 3 |
Because the operand of OP_NOP is Z, OP_NOP does not have any operands. Thus the bytecode is 00.
OP_NOP does nothing, which is called NO-Operation (NOP).
If the instruction begins OP_LOADI, this instruction must have two 8-bit operands. Let assume the operands are 0x02 and 0x04, the bytecode is 03 02 04.
OP_LOADI is a load operation from one register to another. The bytecode 03 02 04 means assign register 04 value into register 02.
If the instruction begins OP_LOADI, this instruction must have one 16-bit operand. Let assume the operand is 0x0005, the bytecode is 25 00 05.
OP_JMP is a jump operation. The next instruction is pointed by the offset, which appears in the 16-bit operand. The bytecode 25 00 05 means that the next instruction is located in offset 0x0005 from current instruction 25 00 05.
Because the offset is signed value, it's necessary to convert from the 16-bit operand to the offset. The converting way is shown later.