From 83f94f6d9de131a308d0cf6ba3c1c508bf695b92 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Sat, 4 Nov 2023 01:21:52 -0500
Subject: [PATCH] feat(cpu): implement HALT

---
 src/core/Scheduler.zig |  4 ++++
 src/core/emu.zig       | 36 +++++++++++++++++++++++++++++++-----
 src/core/nds7/io.zig   | 23 ++++++++++++++---------
 src/core/nds9/Cp15.zig |  4 +++-
 4 files changed, 52 insertions(+), 15 deletions(-)

diff --git a/src/core/Scheduler.zig b/src/core/Scheduler.zig
index 6282e07..a9273d8 100644
--- a/src/core/Scheduler.zig
+++ b/src/core/Scheduler.zig
@@ -41,6 +41,10 @@ pub fn reset(self: *@This()) void {
     self.tick = 0;
 }
 
+pub inline fn peekTimestamp(self: *const @This()) u64 {
+    return self.queue.items[0].tick;
+}
+
 pub inline fn check(self: *@This()) ?Event {
     @setRuntimeSafety(false);
     if (self.tick < self.queue.items[0].tick) return null;
diff --git a/src/core/emu.zig b/src/core/emu.zig
index c3a9b45..460ecb0 100644
--- a/src/core/emu.zig
+++ b/src/core/emu.zig
@@ -107,9 +107,18 @@ pub fn runFrame(scheduler: *Scheduler, system: System) void {
     const frame_end = scheduler.tick + cycles_per_frame;
 
     while (scheduler.tick < frame_end) {
-        system.arm7tdmi.step();
-        system.arm946es.step();
-        system.arm946es.step();
+        switch (isHalted(system)) {
+            .both => scheduler.tick = scheduler.peekTimestamp(),
+            inline else => |halt| {
+                if (comptime halt != .arm9) {
+                    system.arm946es.step();
+                    system.arm946es.step();
+                }
+
+                if (comptime halt != .arm7)
+                    system.arm7tdmi.step();
+            },
+        }
 
         if (scheduler.check()) |ev| {
             const late = scheduler.tick - ev.tick;
@@ -126,6 +135,18 @@ pub fn runFrame(scheduler: *Scheduler, system: System) void {
     }
 }
 
+const Halted = enum { arm7, arm9, both, none };
+
+inline fn isHalted(system: System) Halted {
+    const ret = [_]Halted{ .none, .arm7, .arm9, .both };
+    const nds7_bus: *System.Bus7 = @ptrCast(@alignCast(system.arm7tdmi.bus.ptr));
+
+    const nds9_halt: u2 = @intFromBool(system.cp15.wait_for_interrupt);
+    const nds7_halt: u2 = @intFromBool(nds7_bus.io.haltcnt == .halt);
+
+    return ret[(nds9_halt << 1) | nds7_halt];
+}
+
 // FIXME: Perf win to allocating on the stack instead?
 pub const SharedCtx = struct {
     const MiB = 0x100000;
@@ -320,8 +341,13 @@ pub fn handleInterrupt(comptime dev: Wram.Device, cpu: if (dev == .nds9) *System
     if (!bus_ptr.io.ime or cpu.cpsr.i.read()) return; // ensure irqs are enabled
     if ((bus_ptr.io.ie.raw & bus_ptr.io.irq.raw) == 0) return; // ensure there is an irq to handle
 
-    // TODO: Handle HALT
-    // HALTCNG (NDS7) and CP15 (NDS9)
+    switch (dev) {
+        .nds9 => {
+            const cp15: *System.Cp15 = @ptrCast(@alignCast(cpu.cp15.ptr));
+            cp15.wait_for_interrupt = false;
+        },
+        .nds7 => bus_ptr.io.haltcnt = .execute,
+    }
 
     const ret_addr = cpu.r[15] - if (cpu.cpsr.t.read()) 0 else @as(u32, 4);
     const spsr = cpu.cpsr;
diff --git a/src/core/nds7/io.zig b/src/core/nds7/io.zig
index 01e3853..8778a2c 100644
--- a/src/core/nds7/io.zig
+++ b/src/core/nds7/io.zig
@@ -15,28 +15,26 @@ const log = std.log.scoped(.nds7_io);
 pub const Io = struct {
     shr: *SharedCtx.Io,
 
-    /// Interrupt Master Enable
+    /// IME - Interrupt Master Enable
     /// Read/Write
     ime: bool = false,
 
-    /// Interrupt Enable
+    /// IE -  Interrupt Enable
     /// Read/Write
-    ///
-    /// Caller must cast the `u32` to either `nds7.IntEnable` or `nds9.IntEnable`
     ie: IntEnable = .{ .raw = 0x0000_0000 },
 
     /// IF - Interrupt Request
     /// Read/Write
-    ///
-    /// Caller must cast the `u32` to either `nds7.IntRequest` or `nds9.IntRequest`
     irq: IntRequest = .{ .raw = 0x0000_0000 },
 
-    /// Post Boot Flag
+    /// POSTFLG - Post Boot Flag
     /// Read/Write
-    ///
-    /// Caller must cast the `u8` to either `nds7.PostFlg` or `nds9.PostFlg`
     postflg: PostFlag = .in_progress,
 
+    /// HALTCNT - Low Power Mode Control
+    /// Read/Write
+    haltcnt: Haltcnt = .execute,
+
     pub fn init(io: *SharedCtx.Io) @This() {
         return .{ .shr = io };
     }
@@ -140,4 +138,11 @@ pub const Vramstat = extern union {
     raw: u8,
 };
 
+const Haltcnt = enum(u2) {
+    execute = 0,
+    gba_mode,
+    halt,
+    sleep,
+};
+
 const PostFlag = enum(u8) { in_progress = 0, completed };
diff --git a/src/core/nds9/Cp15.zig b/src/core/nds9/Cp15.zig
index 194ebf4..9c63f26 100644
--- a/src/core/nds9/Cp15.zig
+++ b/src/core/nds9/Cp15.zig
@@ -8,6 +8,8 @@ control: u32 = 0x0005_2078,
 dtcm_size_base: u32 = 0x0300_000A,
 itcm_size_base: u32 = 0x0000_0020,
 
+wait_for_interrupt: bool = false,
+
 // Protection Unit
 // cache_bits_data_unified: u32 = 0x0000_0000,
 // cache_write_bufability: u32 = 0x0000_0000, // For Data Protection Regions
@@ -73,7 +75,7 @@ pub fn write(self: *@This(), op1: u3, cn: u4, cm: u4, op2: u3, value: u32) void
         0b000_0110_0111_001,
         => log.err("TODO: write to PU instruction region #{}", .{cm}),
 
-        0b000_0111_0000_100 => log.err("TODO: halt ARM946E-S", .{}),
+        0b000_0111_0000_100 => self.wait_for_interrupt = true, // NDS9 Halt
         0b000_0111_0101_000 => log.err("TODO: invalidate instruction cache", .{}),
         0b000_0111_0110_000 => log.err("TODO: invalidate data cache", .{}),
         0b000_0111_1010_100 => log.err("TODO: drain write buffer", .{}),