cpu: implement BEQ/BNE/BLT/BGE/BLTU/BGEU

cpu.go

@@ -20,7 +20,7 @@ func NewCPU(w World) CPUState {
 
 func opcode_rd(opcode uint32) uint32  { return (opcode >> 7) & 0b11111 }
 func opcode_rs1(opcode uint32) uint32 { return (opcode >> 15) & 0b11111 } // 2^5 is 32 for 32 registers
-func opcode_rs2(opcode uint32) uint32 { return (opcode >> 19) & 0b11111 }
+func opcode_rs2(opcode uint32) uint32 { return (opcode >> 20) & 0b11111 }
 
 func (c *CPUState) ReadRegister(r uint32) uint32 {
 	if r == 0 {
@@ -61,9 +61,9 @@ func (c *CPUState) Step() error {
 		// Plain unconditional jumps (assembler pseudoinstruction J) are encoded as a JAL with rd=x0.
 
 		var imm uint32 = (opcode >> 12) & 0b11111111111111111111
-		if imm&0b10000000000000000000 != 0 {
+		if imm&0b10000000000000000000 != 0 { // 20th position
 			imm |= 0b11111111111100000000000000000000 // sign extension: if MSB is set in imm, extend with 1's instead of 0's
-		}
+		} // FIXME the encoding seems weirder than this??
 
 		// 2^20 has ~1MiB unsigned range, but it's in multiples of two bytes
 		offset := int32(imm) * 2
@@ -86,7 +86,7 @@ func (c *CPUState) Step() error {
 		// required.
 
 		var imm uint32 = (opcode >> 20) & 0b111111111111
-		if imm&0b100000000000 != 0 {
+		if imm&0b100000000000 != 0 { // 12th position
 			imm |= 0b11111111111111111111000000000000 // sign extension: if MSB is set in imm, extend with 1's instead of 0's
 		}
 
@@ -96,7 +96,70 @@ func (c *CPUState) Step() error {
 
 	case 0b1100011:
 		// BEQ/BNE/BLT/BGE/BLTU/BGEU
-		panic("todo")
+		// All branch instructions use the B-type instruction format. The 12-bit B-immediate encodes signed
+		// offsets in multiples of 2 bytes. The offset is sign-extended and added to the address of the branch
+		// instruction to give the target address. The conditional branch range is ±4 KiB.
+
+		var imm uint32 = (opcode>>6)&0b11111 | (((opcode >> 25) & 0b11111) << 5) // FIXME the encoding seems weirder than this??
+		if imm&0b100000000000 != 0 {                                             // 12th position
+			imm |= 0b11111111111111111111000000000000 // sign extension: if MSB is set in imm, extend with 1's instead of 0's
+		}
+
+		var offset int32 = int32(imm) * 2
+
+		// Branch instructions compare two registers. BEQ and BNE take the branch if registers rs1 and rs2
+		// are equal or unequal respectively. BLT and BLTU take the branch if rs1 is less than rs2, using
+		// signed and unsigned comparison respectively. BGE and BGEU take the branch if rs1 is greater
+		// than or equal to rs2, using signed and unsigned comparison respectively. Note, BGT, BGTU,
+		// BLE, and BLEU can be synthesized by reversing the operands to BLT, BLTU, BGE, and BGEU,
+		// respectively.
+		funct3 := (opcode >> 12) & 0b111
+		switch funct3 {
+		case 0b000:
+			// BEQ
+			if c.ReadRegister(opcode_rs1(opcode)) == c.ReadRegister(opcode_rs2(opcode)) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		case 0b001:
+			// BNE
+			if c.ReadRegister(opcode_rs1(opcode)) != c.ReadRegister(opcode_rs2(opcode)) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		case 0b100:
+			// BLT
+			if int32(c.ReadRegister(opcode_rs1(opcode))) < int32(c.ReadRegister(opcode_rs2(opcode))) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		case 0b101:
+			// BGE
+			if int32(c.ReadRegister(opcode_rs1(opcode))) >= int32(c.ReadRegister(opcode_rs2(opcode))) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		case 0b110:
+			// BLTU
+			if c.ReadRegister(opcode_rs1(opcode)) < c.ReadRegister(opcode_rs2(opcode)) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		case 0b111:
+			// BGEU
+			if c.ReadRegister(opcode_rs1(opcode)) >= c.ReadRegister(opcode_rs2(opcode)) {
+				c.Pc = uint32(int32(c.Pc) + offset)
+				return nil // Don't fallthrough to regular Pc advance
+			}
+
+		default:
+			return ErrInvalidOpcode{}
+		}
 
 	case 0b0000011:
 		// LB/LH/LW/LBU/LHU
@@ -111,7 +174,7 @@ func (c *CPUState) Step() error {
 		funct3 := (opcode >> 12) & 0b111
 
 		var imm uint32 = (opcode >> 20) & 0b111111111111
-		if imm&0b100000000000 != 0 {
+		if imm&0b100000000000 != 0 { // 12th position
 			imm |= 0b11111111111111111111000000000000 // sign extension: if MSB is set in imm, extend with 1's instead of 0's
 		}