S-type

S-type instructions are used for store.

S-Format Instruction Layout

• Mem[Reg[rs1] + offset] <- Reg[rs2]

Immediate in two parts

• Can't have both rs2 and immediate in same place as other instructions

• RISC-V design decision is move low 5 bits of immediate to where rd field was in other instructions - keep rs1/rs2 fields in same place

  • register names more critical than immediate bits in hardware design

I+S Immediate Generation

• Just need a 5-bit mux between two positions where low five bits of immediate can reside in instruction

• Other bits in immediate are wired to fixed positions in instruction

Store Instructions

Adding sw to datapath

How to control the amount of bytes to write to memory?