cpu: implement SB/SH/SW

cpu.go

@@ -238,7 +238,36 @@ func (c *CPUState) Step() error {
 
 	case 0b0100011:
 		// SB/SH/SW
-		panic("todo")
+
+		imm := ((opcode >> 7) & 0b11111) | (((opcode >> 25) & 0b1111111) << 5)
+		memAddr := c.Registers[opcode_rs1(opcode)] + imm
+
+		// The SW, SH, and SB instructions store 32-bit, 16-bit, and 8-bit values from the low bits of register
+		// rs2 to memory.
+		funct3 := (opcode >> 12) & 0b111
+		switch funct3 {
+		case 0b000:
+			// SB
+			err := c.w.WriteByte(memAddr, byte(c.Registers[opcode_rs2(opcode)]))
+			if err != nil {
+				return err
+			}
+
+		case 0b001:
+			// SH
+			err := c.w.Write16(memAddr, uint16(c.Registers[opcode_rs2(opcode)]))
+			if err != nil {
+				return err
+			}
+		case 0b010:
+			// SW
+			err := c.w.Write32(memAddr, c.Registers[opcode_rs2(opcode)])
+			if err != nil {
+				return err
+			}
+		default:
+			return ErrInvalidOpcode{}
+		}
 
 	case 0b0010011:
 		// ADDI/SLTI/SLTIU/XORI/ORI/ANDI/SLLI/SLRI/SRAI