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Update Instruction Set

zxq5
2025-06-16 16:07:18 +01:00
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commit 3ba1db387e
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Instruction-Set.md
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@@ -9,11 +9,7 @@ Also note that immediate (constant/literal) arguments are 16-bits long in I (imm
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| 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. | | 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. | | 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. | 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. |
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. |