diff --git a/generator/spirv/generator.zig b/generator/spirv/generator.zig
index 031bcf4..ea7d97b 100644
--- a/generator/spirv/generator.zig
+++ b/generator/spirv/generator.zig
@@ -3,12 +3,46 @@ const reg = @import("registry.zig");
 const renderSpirv = @import("render.zig").render;
 const Allocator = std.mem.Allocator;
 
+pub fn mergeRegistries(allocator: *Allocator, core: reg.CoreRegistry, extensions: []const reg.ExtensionRegistry) !reg.CoreRegistry {
+    var merged = core;
+
+    var total_instructions = core.instructions.len;
+    var total_operand_kinds = core.operand_kinds.len;
+    for (extensions) |ext| {
+        total_instructions += ext.instructions.len;
+        total_operand_kinds += ext.operand_kinds.len;
+    }
+
+    merged.instructions = try allocator.alloc(reg.Instruction, total_instructions);
+    merged.operand_kinds = try allocator.alloc(reg.OperandKind, total_operand_kinds);
+
+    // Assume the core and extension registries contain no common operand kinds and instructions.
+    for (core.instructions) |inst, i| merged.instructions[i] = inst;
+    for (core.operand_kinds) |operand_kind, i| merged.operand_kinds[i] = operand_kind;
+
+    var inst_i = core.instructions.len;
+    var oper_i = core.operand_kinds.len;
+
+    for (extensions) |ext| {
+        for (ext.instructions) |inst| {
+            merged.instructions[inst_i] = inst;
+            inst_i += 1;
+        }
+
+        for (ext.operand_kinds) |operand_kind| {
+            merged.operand_kinds[oper_i] = operand_kind;
+            oper_i += 1;
+        }
+    }
+
+    return merged;
+}
+
 pub fn generate(allocator: *Allocator, spec_jsons: []const []const u8, writer: anytype) !void {
     var arena = std.heap.ArenaAllocator.init(allocator);
     defer arena.deinit();
 
-    // Only one of the passed specs may be core (and one of them _must_ be core) -
-    // the others must be exensions.
+    // Exactly one of the passed specs must be core - the others must be exensions.
     var core_registry: reg.CoreRegistry = undefined;
     const num_ext_registries = spec_jsons.len - 1;
     const ext_registries = try arena.allocator.alloc(reg.ExtensionRegistry, num_ext_registries);
@@ -34,5 +68,6 @@ pub fn generate(allocator: *Allocator, spec_jsons: []const []const u8, writer: a
         return error.MultipleCoreRegistries;
     }
 
-    try renderSpirv(writer, allocator, &core_registry, ext_registries);
+    const merged = try mergeRegistries(&arena.allocator, core_registry, ext_registries);
+    try renderSpirv(writer, &arena.allocator, &merged);
 }
diff --git a/generator/spirv/render.zig b/generator/spirv/render.zig
index d529d1c..1e75918 100644
--- a/generator/spirv/render.zig
+++ b/generator/spirv/render.zig
@@ -21,8 +21,7 @@ fn Renderer(comptime WriterType: type) type {
 
         writer: WriterType,
         allocator: *Allocator,
-        core: *const reg.CoreRegistry,
-        extensions: []const reg.ExtensionRegistry,
+        registry: *const reg.CoreRegistry,
         id_renderer: IdRenderer,
 
         fn deinit(self: Self) void {
@@ -35,13 +34,12 @@ fn Renderer(comptime WriterType: type) type {
     };
 }
 
-pub fn render(writer: anytype, allocator: *Allocator, core: *const reg.CoreRegistry, extensions: []const reg.ExtensionRegistry) !void {
+pub fn render(writer: anytype, allocator: *Allocator, registry: *const reg.CoreRegistry) !void {
     const id_renderer = IdRenderer.init(allocator, &tags);
     var renderer = Renderer(@TypeOf(writer)) {
         .writer = writer,
         .allocator = allocator,
-        .core = core,
-        .extensions = extensions,
+        .registry = registry,
         .id_renderer = id_renderer,
     };
     defer renderer.deinit();