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/cpu.rs | 58 +++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 41 insertions(+), 17 deletions(-)

(limited to 'src/cpu.rs')

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,
-- 
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