Basic instruction handling, more formatting, step-through mode

src/chip8.rs

@@ -2,13 +2,17 @@
 // Implementation of chip8 machine, registers, and memory makeup 
 
 use std::fmt::Debug;
+use std::process::exit;
+use std::io::{self, Read};
 
-use crate::util;
+use crate::{util, EmulationOpts};
 
 // Memory map constants
 pub const PRGRM_START   : usize = 512;
 pub const MAX_MEM_SIZE  : usize = 4096;
 pub const MAX_PRG_SIZE  : usize = 3584;
+const MAX_NUM_REGS      : usize = 16;
+const MAX_STACK_SIZE    : usize = 16;
 
 // Instruction constants
 const INSTR_NOOP    : u16 = 0x0000;
@@ -17,21 +21,35 @@ const INSTR_CLS:    u16 = 0x00E0;
 const INSTR_RET     : u16 = 0x00EE;
 
 // Commonn Instruction Masks
-const INSTR_MASK_SYS:       u16 = 0x0000;
 const INSTR_MASK_JP             : u16 = 0x1000;
 const INSTR_MASK_CALL           : u16 = 0x2000;
+const INSTR_MASK_SE_VX          : u16 = 0x3000;
+const INSTR_MASK_SNE            : u16 = 0x4000;
+const INSTR_MASK_SE_VX_VY       : u16 = 0x5000;
+const INSTR_MASK_LD_VX          : u16 = 0x6000;
+const INSTR_MASK_ADD_VX         : u16 = 0x7000;
+const INSTR_MASK_LD_VX_VY       : u16 = 0x8000;
+const INSTR_MASK_OR_VX_VY       : u16 = 0x8001;
+const INSTR_MASK_AND_VX_VY      : u16 = 0x8002;
+const INSTR_MASK_XOR_VX_VY      : u16 = 0x8003;
+const INSTR_MASK_ADD_VX_VY      : u16 = 0x8004;
+const INSTR_MASK_SUB_VX_VY      : u16 = 0x8005;
+const INSTR_MASK_SHR_VX_VY      : u16 = 0x8006;
+const INSTR_MASK_SUBN_VX_VY     : u16 = 0x8007;
+const INSTR_MASK_SHL_VX_VY      : u16 = 0x800E;
+const INSTR_MASK_SNE_VX_VY      : u16 = 0x9000;
+const INSTR_MASK_LD_I           : u16 = 0xA000;
+const INSTR_MASK_JP_V0          : u16 = 0xB000;
 
 const ADDR_MASK:  u16 = 0x0FFF;
 
-
-#[derive(Debug)]
 struct EmulationState {
     // Miscellanious registers
     program_counter: u16,
-    stack_ptr: u8,
+    stack_ptr: usize,
     
     // General purpose registers
-    registers: [u8; 16],
+    registers: [u8; MAX_NUM_REGS],
 
     // Memory Map:
     // +---------------+= 0xFFF (4095) End of Chip-8 RAM
@@ -56,10 +74,12 @@ struct EmulationState {
     // | Reserved for  |
     // |  interpreter  |
     // +---------------+= 0x000 (0) Start of Chip-8 RAM
-    memory: [u8; 4096],
+    memory: [u8; MAX_MEM_SIZE],
 
-    // Misc members
+    options: EmulationOpts,
-    debug_enabled: bool
+
+    // Program stack
+    stack: [u16; MAX_STACK_SIZE]
 }
 
 /// # start_emulation
@@ -68,14 +88,15 @@ struct EmulationState {
 ///
 /// * `data` - u8 array of program data.
 /// 
-pub fn start_emulation(data: [u8; MAX_PRG_SIZE], dbg_mode: bool)
+pub fn start_emulation(data: [u8; MAX_PRG_SIZE], options_in: EmulationOpts)
 {
     let mut state: EmulationState = EmulationState {
         program_counter: 0,
         stack_ptr: 0,
         registers: [0; 16],
         memory: [0; MAX_MEM_SIZE],
-        debug_enabled: dbg_mode
+        stack: [0; 16],
+        options: options_in
     };
 
     load_program(data, &mut state);
@@ -137,6 +158,11 @@ fn exec_program(state: &mut EmulationState)
         }
 
         parse_instruction(state, instruction);
+
+        if state.options.step_mode
+        { 
+            pause();
+        }
     }
 }
 
@@ -148,32 +174,82 @@ fn exec_program(state: &mut EmulationState)
 /// * `instr` - Incoming instruction.
 /// 
 fn parse_instruction(state: &mut EmulationState, instr: u16) {
+    // CLS; Clear the display
     if instr == INSTR_CLS
     {
+        // Clear the display
         debug_log_instr(state, instr, "CLS".to_string());
+        // TODO: Implement this
+        return;
     }
 
+    // RET; Return from subroutine
+    //      Sets the program counter to the address at the top of the stack, then pops the value from
+    //      the stack
     if instr == INSTR_RET
     {
         debug_log_instr(state, instr, "RET".to_string());
-    }
         
-    if (instr & INSTR_MASK_SYS) > 0
+        // Halt emulation if stack_ptr is 0, since decrementing on an empty stack
+        // is undefined behavior
+        if state.stack_ptr == 0
         {
-        let addr: u16 = instr & ADDR_MASK;
+            halt(state, instr, format!("Cannot decrement stack pointer below stack limit {}", MAX_STACK_SIZE));
-        debug_log_instr(state, instr, format!("SYS {:04X?}", addr));
         }
 
+        state.stack_ptr -= 1;
+        state.program_counter = state.stack[state.stack_ptr];
+        state.stack[state.stack_ptr] = 0x0000;
+        return;
+    }
+
+    // JP nnn; Jump to location nnn.
+    //      Sets the program counter to nnn.
     if (instr & INSTR_MASK_JP) > 0
     {
         let addr: u16 = instr & ADDR_MASK;
         debug_log_instr(state, instr, format!("JP {:04X?}", addr));
+
+        state.program_counter = addr;
+        return;
     }
 
+    // CALL nnn; Call subroutine at nnn
+    //      Puts the current PC in the stack, then increments the stack pointer. The program counter
+    //      is then set to nnn.
     if (instr & INSTR_MASK_CALL) > 0
     {
         let addr: u16 = instr & ADDR_MASK;
         debug_log_instr(state, instr, format!("CALL {:04X?}", addr));
+
+        // Halt emulation if stack_ptr is increments above stack limit, as this will cause a
+        // stack overflow
+        if (state.stack_ptr + 1) > MAX_STACK_SIZE
+        {
+            halt(state, instr, format!("Cannot increment stack pointer above stack limit {}", MAX_STACK_SIZE));
+        }
+
+        state.stack[state.stack_ptr] = state.program_counter;
+        state.stack_ptr += 1;
+        state.program_counter = addr;
+        return;
+    }
+
+    // SE Vx, byte; Skip next instruction if Vx = kk.
+    //      Compares register Vx to kk, and if they are equal, increments the
+    //      program counter by 2.
+    if (instr & INSTR_MASK_SE_VX) > 0
+    {
+        let bytes:[u8; 2] = instr.to_be_bytes();
+        let register_num: u8 = bytes[0] & 0x0F;
+        let value: u8 = bytes[1] & 0xFF;
+        debug_log_instr(state, instr, format!("SE V{:01X?} {:02X?}", register_num, value));
+
+        if value == state.registers[register_num as usize]
+        {
+            state.program_counter += 2;
+        }
+        return;
     }
 }
 
@@ -187,8 +263,33 @@ fn parse_instruction(state: &mut EmulationState, instr: u16) {
 /// 
 fn debug_log_instr(state: &mut EmulationState, instr: u16, msg: String)
 {
-    if state.debug_enabled
+    if state.options.debug_mode
     {
         println!("PC: {:04?}; INSTR {:04X?}; {}", state.program_counter, instr, msg);
     }
 }
+
+/// # halt
+/// 
+/// Halt the emulator and throw an error
+///
+/// * `msg`     - Message string describing the error
+/// 
+fn halt(state: &mut EmulationState, instr: u16, msg: String)
+{
+    println!("ERROR: EMULATION HALTED\n");
+    println!("\t{}\n", msg);
+    println!("\tPC: {:04?}\t\tINSTR {:04X?}", state.program_counter, instr);
+    exit(0);
+}
+
+/// # Pause
+/// 
+/// Pause the emulation and poll for keyboard input;
+/// Used for stepping over program data 1 instruction at a time
+///
+fn pause() {
+    // Read a single byte and discard
+    let mut stdin = io::stdin();
+    let _ = stdin.read(&mut [0u8]).unwrap();
+}

src/main.rs

@@ -1,4 +1,4 @@
-use core::num;
+use std::option;
 use std::path;
 use std::env;
 use std::fs;
@@ -8,6 +8,14 @@ use std::process::exit;
 mod util;
 mod chip8;
 
+
+#[derive(Default)]
+struct EmulationOpts {
+    debug_mode: bool,
+    step_mode: bool
+}
+
+
 /// # main
 /// 
 /// Main entry point of the emulator
@@ -37,7 +45,7 @@ fn main()
     }
 
     // Runtime options
-    let mut debug_mode: bool = false;
+    let mut emu_options: EmulationOpts = EmulationOpts{..Default::default()};
 
     // Option parsing
     let num_options: usize = args.len() - 1;
@@ -49,7 +57,12 @@ fn main()
             
             for char in flags.chars() {
                 if char == 'd' {
-                    debug_mode = true;
+                    println!("Enabling debug mode...");
+                    emu_options.debug_mode = true;
+                }
+                if char == 's' {
+                    println!("Enabling step-through mode...");
+                    emu_options.step_mode = true;
                 }
             }
         }
@@ -80,7 +93,7 @@ fn main()
             }
 
             println!("Read {} bytes of program data", file_data.len());
-            chip8::start_emulation(program, debug_mode);
+            chip8::start_emulation(program, emu_options);
         }
     } 
     else

src/util.rs

@@ -1,5 +1,8 @@
 pub fn print_help() {
-    print!("Usage: saltnvinegar [CHIP8_FILE]\n");
+    println!("Usage: saltnvinegar [-OPTIONS] [CHIP8_FILE]");
+    println!("Options:");
+    println!("\t-d\tEnable debug output to terminal");
+    println!("\t-s\tEnable step-through-instruction mode");
 }
 
 pub fn u8_tuple_to_u16(u8_in: (u8, u8)) -> u16 {