Sometimes in the course of interactive debugging and testing, your target may crash and become unresponsive at the SwiftX command line. For targets that implement interactive debugging via JTAG (most ARM parts) or BDM (ColdFire parts), you can query the target’s registers and memory to get some idea of what’s happening.
This post presents a simple example and shows how to use these basic tools in SwiftX to figure out what went wrong and get you back on the path to a working project.
To illustrate this, here’s a little program running on an ARM Cortex-M4 core that assigns a memory walk to a background task. It starts at X0 and works its way down through memory. Eventually, it reaches an address that causes an exception.
|U| |S| |R| BACKGROUND TESTER VARIABLE X0 : WALK ( addr1 -- addr2 ) DUP @ DROP CELL - ; : /TESTER ( -- ) TESTER BUILD TESTER ACTIVATE X0 BEGIN WALK AGAIN ;
After calling /TESTER from the command line, we use .S to display the stack, but it just hangs and doesn’t return. At this point, clicking the red stop sign in the toolbar (or selecting “Break” in the Tools menu) will break out of the cross-target link and return to the command line.
We enter the phrase DISCONNECT TARGET HEX to leave us in a state where we’re interacting with words on the host system only with the base set to HEX for entering addresses. Then we use .R (“dot-R”) to examine the processor core’s register context.
/TESTER ok .S Break DISCONNECT TARGET HEX ok R. CPU RUNNING R0 = 20000A20 R1 = 000000FE R2 = 0000000A R3 = 00000001 R4 = 00000000 U/R5 = 20000A24 T/R6 = 1FFEFFFC S/R7 = 2000091C R8 = 00000000 R9 = 00000000 R10 = 00000000 R11 = 00000000 R12 = 00000000 SP/R13 = 20000C28 LR/R14 = FFFFFFFD PC/R15 = 000018A8 PSR = 21000003 PSP = 20000A00 MSP = 20000C28 ok
The Forth word .’ (“dot-tick”) is used to help find out what symbol an address is associated with. In this case, we’ll start with the PC:
18A8 .' <SPIN> ok
The default exception handler in the SwiftX kernel is <SPIN> (a little spin loop), so any unhandled exception or interrupt will end up there.
LABEL <SPIN> BEGIN B END-CODE
Examine the Stack
Let’s examine the return stack to find out how we got there.
The ARMv7-M processor implements two stacks, each with its own hardware stack pointer.
- Main stack (MSP)
- Process stack (PSP)
SwiftX runs in thread mode and uses the PSP as its SP. When an interrupt or exception occurs, the hardware pushes the exception stack frame onto the process stack and then switches to the main stack on entry to the exception handler.
When pushing context to the stack, the hardware saves eight 32-bit words in this order:
- Return address
- LR (R14)
R0 is at the top of the stack (lowest memory address in the stack frame).
We can use the DUMPX memory dump to examine the target’s memory via the JTAG (or BDM for ColdFire targets) to see what the processor core is doing. We’ll start from the PSP up to the address in U, which is always the base of a task’s return stack. Remember that stacks grow downward in memory for this processor.
20000A00 20000A24 OVER - DUMPX 20000A00 20 0A 00 20 FE 00 00 00 0A 00 00 00 01 00 00 00 20000A10 00 00 00 00 35 25 00 00 10 25 00 00 00 02 00 21 20000A20 2B 25 00 00
We can see the exception context on the stack as listed above. Of interest are the return address (2510) and the link register (2535). Bit 0 of LR is the “T” bit in this processor, so the actual address is 2534. Let’s see where those are in our name space:
2534 .' /TESTER +1C ok 2510 .' WALK +04 ok
We can decompile both of those words to verify the addresses.
SEE /TESTER 2518 LR PUSH B500 251A TESTER BL F7FF FFEF 251E BUILD BL F7FF F80F 2522 TESTER BL F7FF FFEB 2526 ACTIVATE BL F7FE FFE1 252A 4 R7 SUBS 3F04 252C 0 R7 R6 STR 603E 252E 20000B24 R6 LDRI 4E02 2530 WALK BL F7FF FFEC 2534 2530 B E7FC ok SEE WALK 250C 4 R7 SUBS 3F04 250E 0 R7 R6 STR 603E 2510 0 R6 R6 LDR 6836 2512 R6 R7 LDM CF40 2514 4 R6 SUBS 3E04 2516 LR BX 4770 ok
The LR address 2534 is just after the call to WALK and the exception itself happened at 2510, the LDR instruction that implements the @ operation in WALK.
These should be enough clues to figure out what caused the crash, fix it, and move forward.