diff --git a/Instruction-Set.md b/Instruction-Set.md index 1579775..adc5cd6 100644 --- a/Instruction-Set.md +++ b/Instruction-Set.md @@ -5,11 +5,25 @@ Below is an overview of the instruction set and the various operands. This table Also note that immediate (constant/literal) arguments are 16-bits long in I (immediate argument) typed instructions. For more information on this, refer to instruction encoding. -### Instruction Format -|Instruction Type| bits 0-7 | bits 8-15 | bits 16-23 | bits 23-31 | -|-|-|-|-|-| -|basic|opcode|operand 1|operand 2|operand 3| -|literal|opcode|value part 1|value part 2|value part 3| +| Type | Description | +| -- | -- | +| R | Used when an instruction takes one or more register arguments, but no immediates. This type is also used by shift and rotation operations, as it contains a 5 bit shift amount field. | +| I | Used when an instruction takes at most two register arguments as well as a halfword immediate argument. This is typically used by immediate arithmetic operations e.g. addi, as well as loads and stores (where a base register and immediate offset are passed). + +Also used by branching instructions. The operand is a signed offset from the current value of PCX. | +| J | Used by jumps excluding jr, which uses a register as its argument. + +Jumps are absolute addresses, but there is a 256MB region around PCX since the argument is 26 bits. + +Since arguments are always word aligned, we bitshift left twice and set the upper 4 bits to match that of the value in PCX. This then forms a valid word-sized address. | + + + + + + + + ### Load/Store and Data Movement Instructions | Hex | Code | Operands | Description |