fix: correctly parse jump bytes

Previously only a single byte was read and saved,
but the J-byte means, that the read signed-byte
must be added to the address of the next
instruction.

src/disasm.rs

@@ -103,6 +103,26 @@ impl Disassembler {
         u16::from_le_bytes([byte1, byte2])
     }
 
+    /// Parse a single byte of binary and advance the offset.
+    /// The returned IByte contains a relative offset to be added to the address
+    /// of the subsequent instruction.
+    pub fn parse_j_byte(&mut self) -> isize {
+        // first interpret as 2-complement, then cast for addition
+        let byte = self.parse_byte() as IByte as isize;
+        let next_addr = (self.offset + 1) as isize;
+        byte + next_addr
+    }
+
+    /// Parse a single byte of binary and advance the offset.
+    /// The returned IByte contains a relative offset to be added to the address
+    /// of the subsequent instruction.
+    pub fn parse_j_word(&mut self) -> isize {
+        // first interpret as 2-complement, then cast for addition
+        let word = self.parse_word() as IWord as isize;
+        let next_addr = (self.offset + 1) as isize;
+        word + next_addr
+    }
+
     /// Takes in a modrm byte and returns mod, reg and r/m.
     fn deconstruct_modrm_byte(modrm: u8) -> (u8, u8, u8) {
         let mode = (modrm >> 6) & 0b11;
@@ -442,22 +462,22 @@ impl Disassembler {
 
                 0x60..=0x6F => return Err(DisasmError::OpcodeUndefined(opcode)),
 
-                0x70 => Mnemonic::JO(self.parse_byte() as IByte),
+                0x70 => Mnemonic::JO(self.parse_j_byte()),
-                0x71 => Mnemonic::JNO(self.parse_byte() as IByte),
+                0x71 => Mnemonic::JNO(self.parse_j_byte()),
-                0x72 => Mnemonic::JB(self.parse_byte() as IByte),
+                0x72 => Mnemonic::JB(self.parse_j_byte()),
-                0x73 => Mnemonic::JNB(self.parse_byte() as IByte),
+                0x73 => Mnemonic::JNB(self.parse_j_byte()),
-                0x74 => Mnemonic::JZ(self.parse_byte() as IByte),
+                0x74 => Mnemonic::JZ(self.parse_j_byte()),
-                0x75 => Mnemonic::JNZ(self.parse_byte() as IByte),
+                0x75 => Mnemonic::JNZ(self.parse_j_byte()),
-                0x76 => Mnemonic::JBE(self.parse_byte() as IByte),
+                0x76 => Mnemonic::JBE(self.parse_j_byte()),
-                0x77 => Mnemonic::JA(self.parse_byte() as IByte),
+                0x77 => Mnemonic::JA(self.parse_j_byte()),
-                0x78 => Mnemonic::JS(self.parse_byte() as IByte),
+                0x78 => Mnemonic::JS(self.parse_j_byte()),
-                0x79 => Mnemonic::JNS(self.parse_byte() as IByte),
+                0x79 => Mnemonic::JNS(self.parse_j_byte()),
-                0x7A => Mnemonic::JPE(self.parse_byte() as IByte),
+                0x7A => Mnemonic::JPE(self.parse_j_byte()),
-                0x7B => Mnemonic::JPO(self.parse_byte() as IByte),
+                0x7B => Mnemonic::JPO(self.parse_j_byte()),
-                0x7C => Mnemonic::JL(self.parse_byte() as IByte),
+                0x7C => Mnemonic::JL(self.parse_j_byte()),
-                0x7D => Mnemonic::JGE(self.parse_byte() as IByte),
+                0x7D => Mnemonic::JGE(self.parse_j_byte()),
-                0x7E => Mnemonic::JLE(self.parse_byte() as IByte),
+                0x7E => Mnemonic::JLE(self.parse_j_byte()),
-                0x7F => Mnemonic::JG(self.parse_byte() as IByte),
+                0x7F => Mnemonic::JG(self.parse_j_byte()),
 
                 // Group 1
                 0x80 => {
@@ -622,36 +642,24 @@ impl Disassembler {
 
                 0xD8..=0xDF => return Err(DisasmError::OpcodeUndefined(opcode)),
 
-                0xE0 => Mnemonic::LOOPNZ(self.parse_byte() as IByte),
+                0xE0 => Mnemonic::LOOPNZ(self.parse_j_byte()),
-                0xE1 => Mnemonic::LOOPZ(self.parse_byte() as IByte),
+                0xE1 => Mnemonic::LOOPZ(self.parse_j_byte()),
-                0xE2 => Mnemonic::LOOP(self.parse_byte() as IByte),
+                0xE2 => Mnemonic::LOOP(self.parse_j_byte()),
-                0xE3 => Mnemonic::JCXZ(self.parse_byte() as IByte),
+                0xE3 => Mnemonic::JCXZ(self.parse_j_byte()),
 
                 0xE4 => Mnemonic::IN_AL(self.parse_byte()),
                 0xE5 => Mnemonic::IN_AX(self.parse_byte()),
                 0xE6 => Mnemonic::OUT_AL(self.parse_byte()),
                 0xE7 => Mnemonic::OUT_AX(self.parse_byte()),
 
-                0xE8 => Mnemonic::CALL_v(self.parse_word()),
+                0xE8 => Mnemonic::CALL_v(self.parse_j_word()),
 
-                // add to address of next instruction
+                0xE9 => Mnemonic::JMP_v(self.parse_j_word()),
-                0xE9 => {
-                    // first interpret as IByte, then cast for addition
-                    let word = self.parse_word() as IByte as isize;
-                    let next_addr = (self.offset + 1) as isize;
-                    Mnemonic::JMP_v(next_addr + word)
-                }
                 0xEA => Mnemonic::JMP_p(Pointer {
                     segment: self.parse_word(),
                     offset: self.parse_word(),
                 }),
-                // add to address of next instruction
+                0xEB => Mnemonic::JMP_b(self.parse_j_byte()),
-                0xEB => {
-                    // first interpret as IByte, then cast for addition
-                    let byte = self.parse_byte() as IByte as isize;
-                    let next_addr = (self.offset + 1) as isize;
-                    Mnemonic::JMP_b(next_addr + byte)
-                }
 
                 0xEC => Mnemonic::IN_ALDX,
                 0xED => Mnemonic::IN_AXDX,

src/instructions.rs

@@ -165,26 +165,26 @@ pub enum Mnemonic {
     DEC_Reg(Register),
     DEC_Mod(ModRmTarget),
     // Jumps
-    JO(IByte),
+    JO(isize),
-    JNO(IByte),
+    JNO(isize),
-    JB(IByte),
+    JB(isize),
-    JNB(IByte),
+    JNB(isize),
-    JZ(IByte),
+    JZ(isize),
-    JNZ(IByte),
+    JNZ(isize),
-    JBE(IByte),
+    JBE(isize),
-    JA(IByte),
+    JA(isize),
-    JS(IByte),
+    JS(isize),
-    JNS(IByte),
+    JNS(isize),
-    JPE(IByte),
+    JPE(isize),
-    JPO(IByte),
+    JPO(isize),
-    JL(IByte),
+    JL(isize),
-    JGE(IByte),
+    JGE(isize),
-    JLE(IByte),
+    JLE(isize),
-    JG(IByte),
+    JG(isize),
-    LOOPNZ(IByte),
+    LOOPNZ(isize),
-    LOOPZ(IByte),
+    LOOPZ(isize),
-    LOOP(IByte),
+    LOOP(isize),
-    JCXZ(IByte),
+    JCXZ(isize),
 
     // TEST
     TEST(ModRmTarget, Register),
@@ -231,7 +231,7 @@ pub enum Mnemonic {
     CWD,
     // CALL
     CALL_p(Pointer),
-    CALL_v(Word),
+    CALL_v(isize),
     CALL_Mod(ModRmTarget),
     // JUMP
     JMP_p(Pointer),