feat(demo): exercise timer setup and IRQ handlers

src/timer_demo.rs

@@ -0,0 +1,53 @@
+//! Original code that configures a timer, handles one IRQ, and returns.
+//! Uses nominal instruction/bus costs; device updates occur at instruction boundaries.
+
+use crate::{
+    cpu::Cpu,
+    machine::Machine,
+    memory::{Memory, MemoryError, BIOS_SIZE, ROM_START},
+};
+
+pub const TIMER_DEMO_STEPS: usize = 21;
+
+pub fn timer_demo() -> Result<Machine, MemoryError> {
+    let rom = [
+        0xe3a0_0301_u32, // MOV r0, #0x04000000
+        0xe280_0c01,     // ADD r0, r0, #0x100 (Timer 0)
+        0xe3e0_100f,     // MVN r1, #15 (low half = 0xfff0)
+        0xe1c0_10b0,     // STRH r1, [r0] (reload)
+        0xe3a0_10c0,     // MOV r1, #0xc0 (enable + local IRQ)
+        0xe3a0_2008,     // MOV r2, #8 (Timer 0 IRQ bit)
+        0xe280_3c01,     // ADD r3, r0, #0x100 (IE)
+        0xe1c3_20b0,     // STRH r2, [r3] (IE)
+        0xe3a0_2001,     // MOV r2, #1
+        0xe583_2008,     // STR r2, [r3, #8] (IME)
+        0xe1c0_10b2,     // STRH r1, [r0, #2] (start timer)
+        0xeaff_fffe,     // B . (waiting for IRQ, not hardware HALT)
+    ]
+    .into_iter()
+    .flat_map(u32::to_le_bytes)
+    .collect();
+
+    // A test vector and handler, not a Nintendo BIOS. The handler deliberately
+    // changes shared r2 and r10; it does not implement the GBA BIOS calling convention.
+    let mut bios = vec![0; BIOS_SIZE];
+    bios[0x18..0x1c].copy_from_slice(&0xea00_0010_u32.to_le_bytes()); // B 0x60
+    for (index, instruction) in [
+        0xe3a0_2000_u32, // MOV r2, #0
+        0xe1c0_20b2,     // STRH r2, [r0, #2] (stop timer)
+        0xe3a0_2008,     // MOV r2, #8
+        0xe1c3_20b2,     // STRH r2, [r3, #2] (acknowledge IF)
+        0xe28a_a001,     // ADD r10, r10, #1
+        0xe25e_f004,     // SUBS pc, lr, #4 (return from IRQ)
+    ]
+    .into_iter()
+    .enumerate()
+    {
+        let offset = 0x60 + index * 4;
+        bios[offset..offset + 4].copy_from_slice(&instruction.to_le_bytes());
+    }
+    Ok(Machine::new(
+        Cpu::new(ROM_START),
+        Memory::with_bios(rom, bios)?,
+    ))
+}