Update Instruction Set

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.
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+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. |
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 ### Load/Store and Data Movement Instructions
 | Hex    | Code | Operands           | Description                                                                                      |