diff --git a/tests/platforms/tec1g/sd-spi-runtime.test.ts b/tests/platforms/tec1g/sd-spi-runtime.test.ts
new file mode 100644
index 0000000..5d2a1a7
--- /dev/null
+++ b/tests/platforms/tec1g/sd-spi-runtime.test.ts
@@ -0,0 +1,110 @@
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
+import { describe, expect, it } from 'vitest';
+import { createTec1gRuntime } from '../../../src/platforms/tec1g/runtime';
+import type { Tec1gPlatformConfigNormalized } from '../../../src/platforms/types';
+
+const MOSI_BIT = 0x01;
+const CLK_BIT = 0x02;
+
+function makeRuntime(image?: Uint8Array) {
+  let sdImagePath: string | undefined;
+  if (image) {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'debug80-sd-'));
+    sdImagePath = path.join(dir, 'sd.img');
+    fs.writeFileSync(sdImagePath, image);
+  }
+  const config: Tec1gPlatformConfigNormalized = {
+    regions: [
+      { start: 0x0000, end: 0x7fff, kind: 'ram' as const },
+      { start: 0xc000, end: 0xffff, kind: 'rom' as const },
+    ],
+    romRanges: [{ start: 0xc000, end: 0xffff }],
+    appStart: 0x0000,
+    entry: 0x0000,
+    updateMs: 100,
+    yieldMs: 0,
+    gimpSignal: false,
+    expansionBankHi: false,
+    matrixMode: false,
+    protectOnReset: false,
+    rtcEnabled: false,
+    sdEnabled: true,
+    sdHighCapacity: true,
+    ...(sdImagePath ? { sdImagePath } : {}),
+  };
+  return createTec1gRuntime(config, () => {});
+}
+
+function writePort(rt: ReturnType<typeof makeRuntime>, value: number): void {
+  rt.ioHandlers.write(0xfd, value & 0xff);
+}
+
+function readPort(rt: ReturnType<typeof makeRuntime>): number {
+  return rt.ioHandlers.read(0xfd) & 0xff;
+}
+
+function pulse(rt: ReturnType<typeof makeRuntime>, bit: number): void {
+  const io = bit ? MOSI_BIT : 0;
+  writePort(rt, io);
+  writePort(rt, io | CLK_BIT);
+  writePort(rt, io);
+}
+
+function writeByte(rt: ReturnType<typeof makeRuntime>, value: number): void {
+  for (let i = 7; i >= 0; i -= 1) {
+    pulse(rt, (value >> i) & 1);
+  }
+}
+
+function readByte(rt: ReturnType<typeof makeRuntime>): number {
+  let value = 0;
+  for (let i = 0; i < 8; i += 1) {
+    writePort(rt, MOSI_BIT);
+    writePort(rt, MOSI_BIT | CLK_BIT);
+    const bit = readPort(rt) & 1;
+    writePort(rt, MOSI_BIT);
+    value = ((value << 1) | bit) & 0xff;
+  }
+  return value & 0xff;
+}
+
+function readResponse(rt: ReturnType<typeof makeRuntime>): number {
+  for (let i = 0; i < 8; i += 1) {
+    const value = readByte(rt);
+    if (value !== 0xff) {
+      return value;
+    }
+  }
+  return 0xff;
+}
+
+function sendCommand(rt: ReturnType<typeof makeRuntime>, bytes: number[]): void {
+  bytes.forEach((byte) => writeByte(rt, byte));
+}
+
+describe('TEC-1G SD SPI runtime', () => {
+  it('initializes and reads a block through port 0xFD', () => {
+    const image = new Uint8Array(1024);
+    image[0x0201] = 0xab;
+    const rt = makeRuntime(image);
+    writePort(rt, 0x00);
+    sendCommand(rt, [0x40, 0x00, 0x00, 0x00, 0x00, 0x95]);
+    expect(readResponse(rt)).toBe(0x01);
+    sendCommand(rt, [0x48, 0x00, 0x00, 0x01, 0xaa, 0x87]);
+    expect(readResponse(rt)).toBe(0x01);
+    sendCommand(rt, [0x77, 0x00, 0x00, 0x00, 0x00, 0x65]);
+    readResponse(rt);
+    sendCommand(rt, [0x69, 0x40, 0x00, 0x00, 0x00, 0x77]);
+    readResponse(rt);
+    sendCommand(rt, [0x7a, 0x00, 0x00, 0x00, 0x00, 0xfd]);
+    expect(readResponse(rt)).toBe(0x00);
+    expect(readByte(rt)).toBe(0x40);
+    sendCommand(rt, [0x51, 0x00, 0x00, 0x00, 0x01, 0xff]);
+    expect(readResponse(rt)).toBe(0x00);
+    expect(readByte(rt)).toBe(0xfe);
+    expect(readByte(rt)).toBe(0x00);
+    expect(readByte(rt)).toBe(0xab);
+  });
+});