From 467ec78e894c8184144bfac039dc7d780abbd050 Mon Sep 17 00:00:00 2001
From: krolxon <krolyxon@tutanota.com>
Date: Thu, 8 Jan 2026 19:21:43 +0530
Subject: add SYS instruction

---
 src/assembler.rs    |  7 +++++++
 src/cpu.rs          | 58 +++++++++++++++++++++++++++++++++++++----------------
 src/instructions.rs |  2 ++
 3 files changed, 50 insertions(+), 17 deletions(-)

(limited to 'src')

diff --git a/src/assembler.rs b/src/assembler.rs
index 73dec00..784337b 100644
--- a/src/assembler.rs
+++ b/src/assembler.rs
@@ -26,6 +26,7 @@ fn is_reg(s: &str) -> bool {
 fn instr_size(tokens: &[String]) -> u16 {
     match tokens[0].as_str() {
         "mov" | "add" | "sub" | "jmp" | "jz" | "jnz" | "cmp" | "mul" | "div" | "call" => 3,
+        "sys" => 2,
         "hlt" | "ret" => 1,
         _ => panic!("Unknown instruction {}", tokens[0]),
     }
@@ -190,6 +191,12 @@ pub fn assembler(source: &str) -> Vec<u8> {
                 bytes.push(Instruction::HLT as u8);
             }
 
+            "sys" => {
+                let imm = tokens[1].parse().unwrap();
+                bytes.push(Instruction::SYS as u8);
+                bytes.push(imm);
+            }
+
             _ => panic!("Line {}: unknown instruction", line_no + 1),
         }
     }
diff --git a/src/cpu.rs b/src/cpu.rs
index 1393e95..4fad73e 100644
--- a/src/cpu.rs
+++ b/src/cpu.rs
@@ -69,21 +69,9 @@ impl CPU {
             x if x == Instruction::CMP_RR as u8 => self.cmp_rr(mem),
             x if x == Instruction::MUL as u8 => self.mul(mem),
             x if x == Instruction::DIV as u8 => self.div(mem),
-            x if x == Instruction::CALL as u8 => {
-                let low  = mem.read(self.pc) as u16; self.inc_pc();
-                let high = mem.read(self.pc) as u16; self.inc_pc();
-                let addr = (high << 8) | low;
-
-                let return_addr = self.pc;
-
-                self.push16(mem, return_addr);
-                self.pc = addr;
-
-            },
-            x if x == Instruction::RET as u8 => {
-                let addr = self.pop16(mem);
-                self.pc = addr;
-            },
+            x if x == Instruction::CALL as u8 => self.call(mem),
+            x if x == Instruction::RET as u8 => self.ret(mem),
+            x if x == Instruction::SYS as u8 => self.syscall(mem),
             x if x == Instruction::HLT as u8 => self.halt(),
             _ => panic!("Unknown opcode {:02X}", opcode),
         }
@@ -94,12 +82,10 @@ impl CPU {
     }
 
     pub fn inc_sp(&mut self) {
-        // self.sp += 1;
         self.sp = self.sp.wrapping_add(1);
     }
 
     pub fn dec_sp(&mut self) {
-        // self.sp -= 1;
         self.sp = self.sp.wrapping_sub(1);
     }
 
@@ -379,6 +365,44 @@ impl CPU {
         // carry unchanged
     }
 
+    pub fn call(&mut self, mem: &mut Memory ) {
+        let low  = mem.read(self.pc) as u16; self.inc_pc();
+        let high = mem.read(self.pc) as u16; self.inc_pc();
+        let addr = (high << 8) | low;
+
+        let return_addr = self.pc;
+
+        self.push16(mem, return_addr);
+        self.pc = addr;
+
+    }
+
+    pub fn ret(&mut self, mem: &mut Memory) {
+        let addr = self.pop16(mem);
+        self.pc = addr;
+    }
+
+
+    pub fn syscall(&mut self, mem: &Memory) {
+        let num = mem.read(self.pc);
+        self.pc = self.pc.wrapping_add(1);
+        match num {
+            0 => {
+                // exit
+                self.halted = true;
+            }
+            1 => {
+                // print A as number
+                println!("{}", self.a);
+            }
+            2 => {
+                // print A as ASCII char
+                print!("{}", self.a as char);
+            }
+            _ => panic!("Unknown syscall {}", num),
+        }
+    }
+
     fn get_reg(&self, r: u8) -> u8 {
         match r {
             0 => self.a,
diff --git a/src/instructions.rs b/src/instructions.rs
index 620b0ad..a0d086f 100644
--- a/src/instructions.rs
+++ b/src/instructions.rs
@@ -16,6 +16,7 @@ pub enum Instruction {
     DIV = 0x0D,
     CALL = 0x0E,
     RET = 0x0F,
+    SYS = 0x10,
     HLT = 0xFF,
 }
 
@@ -33,6 +34,7 @@ impl Instruction {
             0x0D => "DIV",
             0x0E => "CALL",
             0x0F => "RET",
+            0x10 => "SYS",
             0xFF => "HLT",
 
             _ => "???",
-- 
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