Add assembly example code (#80)

* Add assembly example code

This should make it easier for beginners to understand
how to test this emulator.

* cleanup

* fix typo

README.md

@@ -65,6 +65,57 @@ fn main() {
 }
 ```
 
+The same can be achieved, by compiling the euclid example yourself.
+
+First install a 6502 assembler and linker, e.g. [cc65](https://cc65.github.io/cc65/).
+
+```sh
+brew install cc65
+```
+
+Then compile and link the assembly file:
+
+```sh
+cd examples/asm/euclid
+ca65 euclid.a65
+ld65 -C ../linker.cfg -o euclid.bin euclid.o
+```
+
+This will create a binary file `euclid.bin` that you can load into the emulator:
+
+```rust
+use mos6502::memory::Bus;
+use mos6502::memory::Memory;
+use mos6502::cpu;
+use std::fs::read;
+
+fn main() {
+    // Calculate the greatest common divisor of 56 and 49
+    // using Euclid's algorithm.
+    let zero_page_data = [56, 49];
+
+    // Load the binary file from disk
+    let program = match read("examples/asm/euclid/euclid.bin") {
+        Ok(data) => data,
+        Err(err) => {
+            println!("Error reading euclid.bin: {}", err);
+            return;
+        }
+    };
+
+    let mut cpu = cpu::CPU::new(Memory::new());
+
+    cpu.memory.set_bytes(0x00, &zero_page_data);
+    cpu.memory.set_bytes(0x10, &program);
+    cpu.registers.program_counter = 0x10;
+
+    cpu.run();
+
+    // The expected GCD is 7
+    assert_eq!(7, cpu.registers.accumulator);
+}
+```
+
 ## Credits
 
 This started off as a fork of [amw-zero/6502-rs](https://github.com/amw-zero/6502-rs),

examples/asm/euclid/euclid.a65

@@ -0,0 +1,33 @@
+; euclid.a65
+; A program to find the greatest common divisor of two numbers
+
+.ORG $1000
+
+; .algo
+LDA $00    ; Load from F to A
+; .algo_
+sec        ; Set carry flag
+SBC $01    ; Subtract S from the number in A (from F)
+BEQ end    ; Jump to .end if the difference is zero
+BMI swap   ; Jump to .swap if the difference is negative
+STA $00    ; Load A to F
+JMP algo_  ; Jump to .algo_
+
+; .end
+end:
+LDA $00    ; Load from F to A
+BRK        ; Break (end program)
+
+; .swap
+swap:
+LDX $00    ; Load F to X
+LDY $01    ; Load S to Y
+STX $01    ; Store X to S
+STY $00    ; Store Y to F
+JMP algo   ; Jump to .algo
+
+algo:
+JMP algo   ; Infinite loop to prevent program from ending
+
+algo_:
+JMP algo_  ; Infinite loop to prevent program from ending

examples/asm/linker.cfg

@@ -0,0 +1,11 @@
+MEMORY {
+    ZP:     start = $0000, size = $0100, type = rw, define = yes;
+    RAM:    start = $0100, size = $0200, type = rw, define = yes;
+    ROM:    start = $8000, size = $8000, type = ro;
+}
+
+SEGMENTS {
+    ZEROPAGE: load = ZP, type = zp;
+    DATA:     load = RAM, type = rw;
+    CODE:     load = ROM, type = ro;
+}

examples/euclid.rs

@@ -0,0 +1,34 @@
+use mos6502::cpu;
+use mos6502::memory::Bus;
+use mos6502::memory::Memory;
+use std::fs::read;
+
+fn main() {
+    println!("Enter two numbers (< 128) separated by a space to know their GCD.");
+    let mut input = String::new();
+    std::io::stdin().read_line(&mut input).unwrap();
+
+    let zero_page_data = input
+        .split_whitespace()
+        .map(|s| s.parse::<u8>().unwrap())
+        .collect::<Vec<u8>>();
+
+    // Load the binary file from disk
+    let program = match read("examples/asm/euclid/euclid.bin") {
+        Ok(data) => data,
+        Err(err) => {
+            println!("Error reading euclid.bin: {}", err);
+            return;
+        }
+    };
+
+    let mut cpu = cpu::CPU::new(Memory::new());
+
+    cpu.memory.set_bytes(0x00, &zero_page_data);
+    cpu.memory.set_bytes(0x10, &program);
+    cpu.registers.program_counter = 0x10;
+
+    cpu.run();
+
+    println!("GCD is {}", cpu.registers.accumulator);
+}

examples/euclid_bytes.rs

@@ -5,7 +5,7 @@ use mos6502::memory::Bus;
 use mos6502::memory::Memory;
 
 fn main() {
-    println!("Enter two numbers (< 128) to know their GCD:");
+    println!("Enter two numbers (< 128) separated by a space to know their GCD.");
     let mut input = String::new();
     std::io::stdin().read_line(&mut input).unwrap();