From 86418b49a48e7555a9243024dd42a94abc36a7b7 Mon Sep 17 00:00:00 2001
From: Madeline Cronin <maddie.henlo@gmail.com>
Date: Fri, 30 Aug 2024 16:40:18 +0100
Subject: [PATCH] Implemented addition and the generalised parts of handling
 arithmetic ops. In particular, implemented the invert (I) flag, and ensured
 that the program counter is unwritable for arithmetic instructions.

---
 src/processor.rs | 69 +++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 60 insertions(+), 9 deletions(-)

diff --git a/src/processor.rs b/src/processor.rs
index 96e7025..2ee654a 100644
--- a/src/processor.rs
+++ b/src/processor.rs
@@ -2,8 +2,8 @@ pub struct Processor<'a> {
     reg: [u16;32],
     flags: [bool;2], //flag A, then B
     itr_toggle: bool,
-    pub memory: &'a mut [u16],
-    pub disk: &'a mut [u16],
+    pub memory: &'a mut [u16], //Public since memory should be modifiable by the system.
+    pub disk: &'a mut [u16], 
 }
 
 pub fn new<'a>(memory: &'a mut [u16], disk: &'a mut [u16]) -> Processor<'a> {
@@ -18,18 +18,69 @@ pub fn new<'a>(memory: &'a mut [u16], disk: &'a mut [u16]) -> Processor<'a> {
 
 impl <'a> Processor<'a> {
     ///Runs the processor through one instruction.
-    pub fn run(&mut self) {
+    ///Returns false for normal operation, and true to indicate that it should halt.
+    pub fn run(&mut self) -> bool {
         let instruction: (u16,u16) = (self.disk[self.reg[31] as usize],self.disk[self.reg[31] as usize +1]);
         let conditions: ((bool,bool),(bool,bool)) = (( //.0.x is activations, .1.x is conditions
-                (instruction.0 & 0x80) != 0,
-                (instruction.0 & 0x40) != 0),(
-                (instruction.0 & 0x20) != 0,
-                (instruction.0 & 0x10) != 0)
+                (instruction.0 & 0x8000) != 0,
+                (instruction.0 & 0x4000) != 0),(
+                (instruction.0 & 0x2000) != 0,
+                (instruction.0 & 0x1000) != 0)
         );
         if (conditions.0.0 && (conditions.1.0 != self.flags[0])) || (conditions.0.1 && (conditions.1.1 != self.flags[1])) {
-            self.reg[31] += 2; //update the program counter at the end of the instruction
+            self.reg[31] += 2; //update the program counter at the end of instruction
                                //execution.
-            return; //if the conditions are not met, perform no further calculations.
+            return false; //if the conditions are not met, perform no further calculations.
+        }
+
+        match instruction.0 & 0x0F00 {
+            0x0100 => {
+                //Arithmetic operations.
+                let toggles: (bool,bool,bool,bool) = ( //S,C,F,I, see ISA for meanings
+                    (instruction.0 & 0x0080) != 0,
+                    (instruction.0 & 0x0040) != 0,
+                    (instruction.0 & 0x0020) != 0,
+                    (instruction.0 & 0x0010) != 0,
+                );
+                let operands: (usize, usize, usize) = (
+                    ((instruction.1 & 0xF800) >> 11) as usize,
+                    ((instruction.1 & 0x07C0) >> 6) as usize,
+                    ((instruction.1 & 0x004E) >> 1) as usize,
+                );
+                let mut pc_overwrite: Option<u16> = None;
+                if operands.2 == 31 { //program counter is unwritable for non-control flow
+                                      //instructions, but the overflow checks should still occur,
+                                      //so hold onto the next value and replace it later.
+                    pc_overwrite = Some(self.reg[31] + 2);
+                }
+
+                match instruction.0 & 0x000F {
+                    0x0000 => { //Addition
+                        if toggles.1 { //If C=1, then extra logic is required to correctly identify
+                                     //carries
+                            (self.reg[operands.2], self.flags[toggles.2 as usize]) = {
+                                if toggles.0 { //If dealing with signed values, cast the register
+                                               //values before passing them to overflowing_add.
+                                    let (uncast_return, flag) = (self.reg[operands.0] as i16).overflowing_add(self.reg[operands.1] as i16);
+                                    (uncast_return as u16, flag)
+                                }
+                                else { //Otherwise just pass them straight to overflowing_add.
+                                    self.reg[operands.0].overflowing_add(self.reg[operands.1])
+                                }
+                            };
+                        }
+                        else {self.reg[operands.2] = self.reg[operands.0].wrapping_add(self.reg[operands.1])};
+                    }
+                    0x0001 => { //Subtraction
+                        
+                    }
+                    _ => return true
+                }
+                if toggles.3 {self.reg[operands.2] = !self.reg[operands.2]};
+                if let Some(addr) = pc_overwrite {self.reg[31] = addr};
+                return false
+            }
+            _ => return true,
         }
     }
 }
-- 
2.43.4