Add agnes NES emulator (very slow).

Add -W <n> -H <n> width and height options to host-sdl

apps/agnes/build.zig

@@ -0,0 +1,74 @@
+const std = @import("std");
+const CrossTarget = @import("std").zig.CrossTarget;
+const Target = @import("std").Target;
+const Feature = @import("std").Target.Cpu.Feature;
+
+pub fn build(b: *std.Build) void {
+    var options = b.addOptions();
+    const heapsize = b.option(u32, "heapsize", "heap size in bytes") orelse 0;   // -Dheapsize=u32
+    options.addOption(u32, "heapsize", heapsize);
+
+    const features = Target.riscv.Feature;
+    var disabled_features = Feature.Set.empty;
+    var enabled_features = Feature.Set.empty;
+
+    // disable all CPU extensions
+    disabled_features.addFeature(@intFromEnum(features.a));
+    disabled_features.addFeature(@intFromEnum(features.c));
+    disabled_features.addFeature(@intFromEnum(features.d));
+    disabled_features.addFeature(@intFromEnum(features.e));
+    disabled_features.addFeature(@intFromEnum(features.f));
+    // except multiply
+    enabled_features.addFeature(@intFromEnum(features.m));
+
+    const target = b.resolveTargetQuery(.{
+        .cpu_arch = Target.Cpu.Arch.riscv32,
+        .os_tag = Target.Os.Tag.freestanding,
+        .abi = Target.Abi.none,
+        .cpu_model = .{ .explicit = &std.Target.riscv.cpu.generic_rv32},
+        .cpu_features_sub = disabled_features,
+        .cpu_features_add = enabled_features
+    });
+
+    const exe = b.addExecutable(.{
+        .name = "agnes",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = .ReleaseFast,
+        }),
+    });
+    exe.root_module.addOptions("buildopts", options);
+
+    // add zeptolibc
+    const zeptolibc_dep = b.dependency("zeptolibc", .{
+        .target = target,
+        .optimize = .ReleaseFast,
+    });
+    exe.root_module.addImport("zeptolibc", zeptolibc_dep.module("zeptolibc"));
+    exe.root_module.addIncludePath(zeptolibc_dep.path("include"));
+    exe.root_module.addIncludePath(zeptolibc_dep.path("include/zeptolibc"));
+
+    exe.addCSourceFiles(.{ 
+        .files = &.{
+            "src/agnes.c",
+        },
+        .flags = &.{ "-Wall", "-fno-sanitize=undefined" }
+    });
+    exe.addIncludePath(b.path("src/"));
+
+    b.installArtifact(exe);
+
+    exe.addAssemblyFile(b.path("../common/crt0.S"));
+    exe.setLinkerScript(b.path("../common/linker.ld"));
+    exe.addIncludePath(b.path("../../common"));
+    exe.addIncludePath(b.path("../common"));
+
+    const bin = b.addObjCopy(exe.getEmittedBin(), .{
+        .format = .bin,
+    });
+    bin.step.dependOn(&exe.step);
+
+    const copy_bin = b.addInstallBinFile(bin.getOutput(), "agnes.bin");
+    b.default_step.dependOn(&copy_bin.step);
+}