How do engineers peek inside a running chip?
JTAG is a standard test port on most chips. Through it, engineers' debug tools can pause the chip, look inside and find bugs.
1. A test port on the chip
JTAG is a standard test port on most chips, first made to check the wiring on circuit boards. Today a debug adapter also links it to a laptop to find bugs, mistakes in the chip or its programs.
2. Pause, step, play
Through the port, debug tools can pause a processor at any moment, like pausing a video. Then engineers run the program one step at a time and watch what changes.
3. Look inside, even change it
While the chip is paused, engineers read its registers (tiny storage slots inside it) and memory. Unlike a video, they can also change the values, then press play.
4. Locked on store chips
On chips sold in stores, the port is usually locked for security. Only authorized engineers can unlock it.
5. When the system freezes
If the operating system hangs, the screen freezes like a stuck video. The JTAG port and other side-band links do not need the operating system, so they can still read the chip's status.
Words to know
- JTAG
- A standard test and debug port, named after the group that wrote the standard.
- Debug adapter
- A small box that connects a computer to a chip's test port.
- Register
- A tiny, very fast storage slot inside a processor.
- Side-band link
- A separate connection that reads a chip's status without the operating system.
- Hang
- When a computer stops responding, but does not switch off.
Where you meet it at work
When a test system hangs, engineers use a debug adapter on the JTAG port to see what the chip was doing.
Check yourself
What can engineers do while a chip is paused through its JTAG port?Think first, then tap to see the answer.
Read and change its registers and memory, then run the program one step at a time.