ft(interpreter): impl SAR

src/interpreter/computer.rs

@@ -1,6 +1,5 @@
 use clap::Parser;
 use core::fmt;
-use std::path::Path;
 
 use crate::{
     Args,
@@ -463,29 +462,59 @@ impl Computer {
         Ok(imm)
     }
 
-    /// Shift bits of data, pointed to by a [`ModRMTarget`]
+    /// Shift bits of data, pointed to by a [`ModRMTarget`].
+    /// SHL: Direction::Left, false
+    /// SHR: Direction::Right, false
+    /// SAR: Direction::Right, true
     pub fn shift(
         &mut self,
         target: ModRmTarget,
         shift: ImmediateOperand,    // how many shifts
         shift_direction: Direction, // direction of shifting
+        arithmetic_right: bool,     // arithmetic shift yes/no
     ) -> Result<(), InterpreterError> {
         let result = match shift_direction {
             Direction::Left => {
                 let imm = self.read_modrm(target)? << (shift - 1);
+
+                // CF is last bit, which is shifted out
                 self.flags.cf = imm.msb();
                 let result = imm << 1;
+
+                // OF only affected for 1 bit shifts
+                if shift == 1_u8.into() {
                     self.flags.of = if result.msb() == self.flags.cf {
                         false
                     } else {
                         true
                     };
+                }
+
                 result
             }
             Direction::Right => {
-                let imm = self.read_modrm(target)? >> (shift - 1);
-                self.flags.cf = imm.msb();
-                imm >> 1
+                let target_val = self.read_modrm(target)?;
+                let imm = target_val >> (shift - 1);
+
+                // OF only affected for 1 bit shifts
+                if shift == 1_u8.into() {
+                    if arithmetic_right {
+                        self.flags.of = false;
+                    } else {
+                        self.flags.of = target_val.msb();
+                    }
+                }
+
+                // CF is last bit, which is shifted out
+                self.flags.cf = imm.lsb();
+                let result = imm >> 1;
+
+                // preserve sign for SAR
+                if arithmetic_right {
+                    result.set_sign(imm.msb())
+                } else {
+                    result
+                }
             }
         };
         self.write_modrm(target, result)?;

src/interpreter/interpreter.rs

@@ -770,35 +770,37 @@ impl Interpreter {
                  * Shfit
                  */
                 Mnemonic::SHL_b(target, b) => {
-                    self.computer.shift(target, b.into(), Direction::Left)?
+                    self.computer
+                        .shift(target, b.into(), Direction::Left, false)?
                 }
                 Mnemonic::SHR_b(target, b) => {
-                    self.computer.shift(target, b.into(), Direction::Right)?;
-                    // For the SHR instruction, the OF flag is set to the most-significant bit of the original operand.
-                    self.computer.flags.of = self.computer.read_modrm(target)?.msb();
+                    self.computer
+                        .shift(target, b.into(), Direction::Right, false)?;
                 }
-                Mnemonic::SAR_b(_, _) => todo!(),
-                // Mnemonic::SAR_b(_, _) => {
-                // For the SAR instruction, the OF flag is cleared for all 1-bit shifts.
-                //     if b == 1 {
-                //         self.flags.of = 0;
-                //     }
-                // },
+                Mnemonic::SAR_b(target, b) => {
+                    self.computer
+                        .shift(target, b.into(), Direction::Right, true)?
+                }
+
                 Mnemonic::SHL_fromReg(target, reg) => self.computer.shift(
                     target,
                     self.computer.regs.read(reg).into(),
                     Direction::Left,
+                    false,
                 )?,
-                Mnemonic::SHR_fromReg(target, reg) => {
-                    self.computer.shift(
+                Mnemonic::SHR_fromReg(target, reg) => self.computer.shift(
                     target,
                     self.computer.regs.read(reg).into(),
                     Direction::Right,
-                    )?;
-                    // For the SHR instruction, the OF flag is set to the most-significant bit of the original operand.
-                    self.computer.flags.of = self.computer.read_modrm(target)?.msb();
-                }
-                Mnemonic::SAR_fromReg(_, _) => todo!(),
+                    false,
+                )?,
+
+                Mnemonic::SAR_fromReg(target, reg) => self.computer.shift(
+                    target,
+                    self.computer.regs.read(reg).into(),
+                    Direction::Right,
+                    true,
+                )?,
 
                 /*
                  * IN

src/operands.rs

@@ -9,7 +9,7 @@ use crate::{disasm::DisasmError, register::Register};
 use core::fmt;
 use std::{
     cmp::Ordering,
-    ops::{Add, BitAnd, BitOr, BitXor, Div, Mul, Not, Shl, Shr, Sub},
+    ops::{Add, BitAnd, BitOr, BitXor, Div, Not, Shl, Shr, Sub},
 };
 
 pub type Byte = u8; // b
@@ -72,6 +72,20 @@ impl ImmediateOperand {
         }
     }
 
+    /// Sets or removes sign.
+    pub fn set_sign(self, sign: bool) -> Self {
+        match self {
+            Self::Byte(b) => {
+                let msb = 1 << 7;
+                Self::Byte(if sign { b | msb } else { b & !msb })
+            }
+            Self::Word(w) => {
+                let msb = 1 << 15;
+                Self::Word(if sign { w | msb } else { w & !msb })
+            }
+        }
+    }
+
     /// Check if inner value is zero.
     pub fn zero(&self) -> bool {
         match self {
@@ -92,7 +106,7 @@ impl ImmediateOperand {
     }
 
     /// Check if least significant bit is set.
-    pub fn _lsb(&self) -> bool {
+    pub fn lsb(&self) -> bool {
         match self {
             Self::Byte(byte) => return byte & 1 == 1,
             Self::Word(word) => return word & 1 == 1,