Fix a bug with the branch instructions not being able to move backward (offsets were unsigned)

.vscode/launch.json

@@ -7,6 +7,7 @@
 
 
 
+
         {
             "type": "lldb",
             "request": "launch",
@@ -59,7 +60,7 @@
                     "kind": "bin"
                 }
             },
-            "args": ["simple_test_machine\\programs\\mult10.bin"],
+            "args": ["simple_test_machine\\programs\\hello.bin"],
             "cwd": "${workspaceFolder}"
         },
     ]

simple_test_machine/programs/hello.asm

@@ -1,37 +1,31 @@
-        # Load string into memory at the output address
+
-        0xA2, b'H',             # LDX H
+; hello world example
-        0x86, output_addr,      # STX
+; for win2c64 by Aart Bik
-        0xA2, b'e',             # LDX e
+; http://www.aartbik.com/
-        0x86, output_addr + 1,  # STX
+
-        0xA2, b'l',             # LDX l
+; Adapted for the RE6502 emulator simple test machine
-        0x86, output_addr + 2,  # STX
+
-        0xA2, b'l',             # LDX l
+strout          .equ   $00A0   ; console output address
-        0x86, output_addr + 3,  # STX
+print_flag      .equ   $009E   ; console output address
-        0xA2, b'o',             # LDX o
+
-        0x86, output_addr + 4,  # STX
+main    .org   $0200   ; program load address for the simple test machine
-
+        ldx    #0
-        0xA2, b' ',             # LDX ' '
+loop    lda    text,x
-        0x86, output_addr + 5,  # STX
+        sta    strout,x    
-
+        inx
-        0xA2, b'w',             # LDX w
+        cpx    #11
-        0x86, output_addr + 6,  # STX
+        bne    loop 
-        0xA2, b'o',             # LDX o
+
-        0x86, output_addr + 7,  # STX
+        ; null terminate the string
-        0xA2, b'r',             # LDX r
+        lda #0           
-        0x86, output_addr + 8,  # STX
+        sta strout,x          
-        0xA2, b'l',             # LDX l
+
-        0x86, output_addr + 9,  # STX
+        ; Set flag to do the print
-        0xA2, b'd',             # LDX d
+        ldx #1           ; 0xA2, 0x01,  
-        0x86, output_addr + 10, # STX
+        stx print_flag   ; 0x86, 0x9E, ; Print string flag is at 0x9E
-        0xA2, b'!',             # LDX !
+
-        0x86, output_addr + 11, # STX
+        ; End the program
-
+        rts            ; 0x60 
-        0xA2, 0x00,             # LDX 0
+
-        0x86, output_addr + 12, # STX
+        ; Variables
-
+text    .byte  "HELLO WORLD"
-        # Set flag to do the print
-        0xA2, 0x01,             # LDX 1
-        0x86, print_flag_addr,  # STX
-
-        # End the program
-        0x60                    # RTS

simple_test_machine/programs/mult10.asm

@@ -1,5 +1,7 @@
 
 ; Multiply 7 with 10 and print the result
+; For testing this program was assembled with c64 (using the -R option):
+; https://www.aartbik.com/MISC/c64.html
 
         ; FAST MULTIPLY program from:
         ; http://6502.org/source/integers/fastx10.htm
@@ -12,15 +14,16 @@ main    LDA #7      ; load 7 into the accumulator
         ADC TEMP    ;as result, A = x*8 + x*2
 
         ; PRINT RESULT
-        STA $A0          ; 0x85, 0xA0, ; store A into the output addr
+        STA $A0          ; 0x85, 0xA0, ; store A into the console output address
         LDX #0           ; 0xA2, 0x00, ; null terminator
         STX $A1          ; 0x86, 0xA1, ; store null terminator to output addr + 1
 
         ; Set flag to do the print
         LDX #1           ; 0xA2, 0x01,  
-        STX $9F          ; 0x86, 0x9F, 
+        STX $9F          ; 0x86, 0x9F, ; Print byte flag is at 0x9F
 
         ; End the program
         RTS            ; 0x60 
 
+        ; Variables
         TEMP    .byte 0

src/r6502/instructions.rs

@@ -581,7 +581,7 @@ impl Instructions
         }
 
         cpu.clear_flag(Flags::N);
-        if (cpu.x as u16 - cpu.working_data) & 0x80 > 0
+        if (cpu.x as i8 - cpu.working_data as i8) as u8 & 0x80 > 0
         {
             cpu.set_flag(Flags::N);
         }
@@ -596,7 +596,12 @@ impl Instructions
     {
         if cpu.check_flag(Flags::N) == 0
         {
-            cpu.pc += cpu.working_data;
+            // The branch offset can be negative so we need to do some
+            // type juggling to make that work
+            let offset = cpu.working_data as i8;         // allow the value to be negative
+            let offset_wide = offset as i16;            // Expand to match the 2 byte pc while retaning the possible negative sign
+            let new_pc = cpu.pc as i16 + offset_wide;   // Add the offset
+            cpu.pc = new_pc as u16;                          // Store the offset
         }
     }	
 
@@ -604,7 +609,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::N) != 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }	
 
@@ -612,7 +620,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::V) == 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }	
 
@@ -620,7 +631,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::V) != 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }	
 
@@ -628,7 +642,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::C) == 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }	
 
@@ -636,7 +653,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::C) != 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     } 
 
@@ -644,7 +664,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::Z) == 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }	
 
@@ -652,7 +675,10 @@ impl Instructions
     {
         if cpu.check_flag(Flags::Z) != 0
         {
-            cpu.pc += cpu.working_data;
+            let offset = cpu.working_data as i8;
+            let offset_wide = offset as i16;
+            let new_pc = cpu.pc as i16 + offset_wide;
+            cpu.pc = new_pc as u16;
         }
     }
 

todo/todo.todo

@@ -1,4 +1,9 @@
 
+BUGS:
+    ✔ Branch instructions seem to jump to the wrong place @done(24-01-22 19:08)
+        - looks like the offset should be able to be a negative value but it's not
+
+
 General:
     ✔ Add unit tests for each instruction and address mode @done(23-11-07 19:57)
     ☐ Fully implement clock cycle tracking