fix: implement proper SRAM mirroring and stub Flash

src/Bus.zig

@@ -147,7 +147,7 @@ pub fn read8(self: *const Self, addr: u32) u8 {
         0x0800_0000...0x09FF_FFFF => self.pak.get8(addr - 0x0800_0000),
         0x0A00_0000...0x0BFF_FFFF => self.pak.get8(addr - 0x0A00_0000),
         0x0C00_0000...0x0DFF_FFFF => self.pak.get8(addr - 0x0C00_0000),
-        0x0E00_0000...0x0EFF_FFFF => self.pak.sram.get8(addr & 0xFFFF),
+        0x0E00_0000...0x0E00_FFFF => self.pak.backup.get8(addr & 0xFFFF),
 
         else => undRead("Tried to read from 0x{X:0>2}", .{addr}),
     };
@@ -162,7 +162,7 @@ pub fn write8(self: *Self, addr: u32, byte: u8) void {
         0x0400_0410 => log.info("Ignored write of 0x{X:0>2} to 0x{X:0>8}", .{ byte, addr }),
 
         // External Memory (Game Pak)
-        0x0E00_0000...0x0E00_FFFF => self.pak.sram.set8(addr & 0xFFFF, byte),
+        0x0E00_0000...0x0E00_FFFF => self.pak.backup.set8(addr & 0xFFFF, byte),
         else => undWrite("Tried to write 0x{X:0>2} to 0x{X:0>8}", .{ byte, addr }),
     }
 }

src/bus/GamePak.zig

@@ -1,6 +1,6 @@
 const std = @import("std");
 
-const Sram = @import("Sram.zig");
+const Backup = @import("backup.zig").Backup;
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.GamePak);
 const Self = @This();
@@ -8,7 +8,7 @@ const Self = @This();
 title: [12]u8,
 buf: []u8,
 alloc: Allocator,
-sram: Sram,
+backup: Backup,
 
 pub fn init(alloc: Allocator, path: []const u8) !Self {
     const file = try std.fs.cwd().openFile(path, .{});
@@ -18,11 +18,13 @@ pub fn init(alloc: Allocator, path: []const u8) !Self {
     const buf = try file.readToEndAlloc(alloc, len);
     const title = parseTitle(buf);
 
+    const kind = Backup.guessKind(buf) orelse .Sram;
+
     const pak = Self{
         .buf = buf,
         .alloc = alloc,
         .title = title,
-        .sram = try Sram.init(alloc),
+        .backup = try Backup.init(alloc, kind),
     };
     pak.parseHeader();
 
@@ -55,7 +57,7 @@ fn lookupMaker(slice: *const [2]u8) ?[]const u8 {
 
 pub fn deinit(self: Self) void {
     self.alloc.free(self.buf);
-    self.sram.deinit();
+    self.backup.deinit();
 }
 
 pub fn get32(self: *const Self, idx: usize) u32 {

src/bus/Sram.zig

@@ -1,31 +0,0 @@
-const std = @import("std");
-
-const Allocator = std.mem.Allocator;
-const log = std.log.scoped(.SRAM);
-const Self = @This();
-
-buf: []u8,
-alloc: Allocator,
-
-pub fn init(alloc: Allocator) !Self {
-    // FIXME: SRAM is more than just a 64KB block of memory
-    const buf = try alloc.alloc(u8, 0x10000);
-    std.mem.set(u8, buf, 0);
-
-    return Self{
-        .buf = buf,
-        .alloc = alloc,
-    };
-}
-
-pub fn deinit(self: Self) void {
-    self.alloc.free(self.buf);
-}
-
-pub fn get8(self: *const Self, idx: usize) u8 {
-    return self.buf[idx];
-}
-
-pub fn set8(self: *Self, idx: usize, byte: u8) void {
-    self.buf[idx] = byte;
-}

src/bus/backup.zig

@@ -0,0 +1,98 @@
+const std = @import("std");
+
+const Allocator = std.mem.Allocator;
+const log = std.log.scoped(.Backup);
+
+const backup_kinds = [5]Needle{
+    .{ .str = "EEPROM_V", .kind = .Eeprom },
+    .{ .str = "SRAM_V", .kind = .Sram },
+    .{ .str = "FLASH_V", .kind = .Flash },
+    .{ .str = "FLASH512_V", .kind = .Flash },
+    .{ .str = "FLASH1M_V", .kind = .Flash1M },
+};
+
+pub const Backup = struct {
+    const Self = @This();
+
+    buf: []u8,
+    alloc: Allocator,
+    kind: BackupKind,
+
+    pub fn init(alloc: Allocator, kind: BackupKind) !Self {
+        const buf_len: usize = switch (kind) {
+            .Sram => 0x8000, // 32K
+            .Flash => 0x10000, // 64K
+            .Flash1M => 0x20000, // 128K
+            .Eeprom => 0x2000, // FIXME: We assume 8K here
+        };
+
+        const buf = try alloc.alloc(u8, buf_len);
+        std.mem.set(u8, buf, 0);
+
+        return Self{
+            .buf = buf,
+            .alloc = alloc,
+            .kind = kind,
+        };
+    }
+
+    pub fn guessKind(rom: []const u8) ?BackupKind {
+        @setRuntimeSafety(false);
+
+        for (backup_kinds) |needle| {
+            const needle_len = needle.str.len;
+
+            var i: usize = 0;
+            while ((i + needle_len) < rom.len) : (i += 1) {
+                if (std.mem.eql(u8, needle.str, rom[i..][0..needle_len])) return needle.kind;
+            }
+        }
+
+        return null;
+    }
+
+    pub fn deinit(self: Self) void {
+        self.alloc.free(self.buf);
+    }
+
+    pub fn get8(self: *const Self, idx: usize) u8 {
+        // TODO: Implement Flash and EEPROM
+        switch (self.kind) {
+            .Flash => return switch (idx) {
+                0x0000 => 0x32, // Panasonic manufacturer ID
+                0x0001 => 0x1B, // Panasonic device ID
+                else => self.buf[idx],
+            },
+            .Flash1M => return switch (idx) {
+                0x0000 => 0x62, // Sanyo manufacturer ID
+                0x0001 => 0x13, // Sanyo device ID
+                else => self.buf[idx],
+            },
+            .Eeprom => return self.buf[idx],
+            .Sram => return self.buf[idx & 0x7FFF], // 32K SRAM chips are repeated
+        }
+    }
+
+    pub fn set8(self: *Self, idx: usize, byte: u8) void {
+        self.buf[idx] = byte;
+    }
+};
+
+const BackupKind = enum {
+    Eeprom,
+    Sram,
+    Flash,
+    Flash1M,
+};
+
+const Needle = struct {
+    str: []const u8,
+    kind: BackupKind,
+
+    fn init(str: []const u8, kind: BackupKind) @This() {
+        return .{
+            .str = str,
+            .kind = kind,
+        };
+    }
+};