Add BRGE and BRLT branch instructions (#10)

Implemented op_brge and op_brlt methods for signed conditional branching in the CPU class. Updated the README to document these instructions and added unit tests to verify correct control flow behavior.

Author: sql-hkr

README.md

@@ -221,7 +221,7 @@ Below is a concise, categorized summary of the Tiny8 instruction set (mnemonics
     - JMP label / RJMP offset — unconditional jump
     - CALL label / RCALL offset — call subroutine (push return address)
     - RET / RETI — return from subroutine / return from interrupt (sets I)
-    - BRNE / BREQ / BRCS / BRCC — conditional branches based on flags
+    - BRNE / BREQ / BRCS / BRCC / BRGE / BRLT — conditional branches based on flags
     - CP Rd, Rr / CPI Rd, K — compare (sets flags)
     - CPSE Rd, Rr — compare and skip if equal
 

src/tiny8/cpu.py

@@ -549,7 +549,19 @@ def op_out(self, port: int, rr: int):
         val = self.read_reg(rr)
         self.write_ram(port, val)
 
-    def op_jmp(self, label: str):
+    def op_jmp(self, label: str | int):
+        """Jump to a given label or numeric address by updating the program counter.
+
+        This operation sets the CPU's program counter (self.pc) to the target address minus one.
+        The subtraction of one accounts for the fact that the instruction dispatcher will typically
+        increment the program counter after the current instruction completes.
+
+        Args:
+            label (str | int): The jump target. If a string, it is treated as a symbolic label
+                and looked up in self.labels to obtain its numeric address. If an int (or any
+                value convertible to int), it is used directly as the numeric address.
+        """
+
         if isinstance(label, str):
             if label not in self.labels:
                 raise KeyError(f"Label {label} not found")
@@ -893,6 +905,28 @@ def op_brcc(self, label: str):
         if not c:
             self.op_jmp(label)
 
+    def op_brge(self, label: str | int):
+        """BRGE - Branch if Greater or Equal (Signed)
+
+        Args:
+            label: Destination label or address to jump to if the condition is met.
+        """
+
+        s = self.get_flag(SREG_S)
+        if not s:
+            self.op_jmp(label)
+
+    def op_brlt(self, label: str | int):
+        """BRLT - Branch if Less Than (Signed).
+
+        Args:
+            label: Destination label or address to jump to if the condition is met.
+        """
+
+        s = self.get_flag(SREG_S)
+        if s:
+            self.op_jmp(label)
+
     def op_push(self, rr: int):
         """Push a register value onto the stack.
 

tests/test_control_flow.py

@@ -98,3 +98,39 @@ def test_cpse_skips_next(self):
         # If cpse skipped the next instruction, r6 should remain zero and r7 should be loaded
         self.assertEqual(cpu.read_reg(6), 0x00)
         self.assertEqual(cpu.read_reg(7), 0xBB)
+
+    def test_brlt(self):
+        # BRLT: branch when signed (Rd < Rr). Use -1 (0xFF) < 1 (0x01)
+        src = """
+            ldi r0, $FF    ; -1
+            ldi r1, $01    ; 1
+            cp r0, r1
+            brlt less
+            ldi r3, $01
+            jmp done
+        less:
+            ldi r3, $02
+        done:
+            nop
+        """
+        cpu = self.run_asm(src)
+        # -1 < 1 so branch taken -> r3 == 2
+        self.assertEqual(cpu.read_reg(3), 0x02)
+
+    def test_brge(self):
+        # BRGE: branch when signed (Rd >= Rr). Use 1 >= -1
+        src2 = """
+            ldi r0, $01    ; 1
+            ldi r1, $FF    ; -1
+            cp r0, r1
+            brge ge
+            ldi r3, $01
+            jmp done
+        ge:
+            ldi r3, $02
+        done:
+            nop
+        """
+        cpu2 = self.run_asm(src2)
+        # 1 >= -1 so branch taken -> r3 == 2
+        self.assertEqual(cpu2.read_reg(3), 0x02)