Software hanging at B.N instruction in disassembler -- No idea what it means

Question

I'm trying to debug some software I've written for the NXP LPC1850 evaluation board. At the moment, when I halt my code I cannot see where it at in my C code, but I can in the disassembler window. The line it is hanging on is at address 0x40. The instruction is B.N . I've looked it up on the ARM Information Center but I don't know anything about assembler and I'm afraid that even after reading around it means nothing to me. ``` 0x0: 0x9ff0 LDR R7, [SP, #0x3c0] 0x2: 0x1008 ASRS R0, R1, #32 0x4: 0x2bad CMP R3, #173 ; 0xad 0x6: 0x1040 ASRS R0, R0, #1 0x8: 0x0041 LSLS R1, R0, #1 0xa: 0x1040 ASRS R0, R0, #1 0xc: 0x0043 LSLS R3, R0, #1 0xe: 0x1040 ASRS R0, R0, #1 0x10: 0x0045 LSLS R5, R0, #1 0x12: 0x1040 ASRS R0, R0, #1 0x14: 0x0047 LSLS R7, R0, #1 0x16: 0x1040 ASRS R0, R0, #1 0x18: 0x0049 LSLS R1, R1, #1 0x1a: 0x1040 ASRS R0, R0, #1 0x1c: 0x0000 MOVS R0, R0 0x1e: 0x0000 MOVS R0, R0 0x20: 0x0000 MOVS R0, R0 0x22: 0x0000 MOVS R0, R0 0x24: 0x0000 MOVS R0, R0 0x26: 0x0000 MOVS R0, R0 0x28: 0x0…

Accepted Answer

In Cortex-M3, the lowest addresses are not code, but the vector table. For example, entry at 0 is the reset value of `SP`, at 4 is the reset `PC`, and the following entries are various exceptions. If we reinterptet the dump as data, we get the following:

```
00 DCD 0x10089FF0
04 DCD 0x10402BAD
08 DCD 0x10400041
0C DCD 0x10400043
10 DCD 0x10400045
14 DCD 0x10400047
18 DCD 0x10400049
1C DCD 0
20 DCD 0
24 DCD 0
28 DCD 0
2C DCD 0x1040004B
30 DCD 0x1040004D
34 DCD 0
38 DCD 0x1040004F
3C DCD 0x10401645
```

It becomes obvious that the code has been linked to run from 10400000. If we take that into account, and convert integers to offsets, we get:

```
00  DCD 0x10089FF0          ; Initial SP
04  DCD 0x10402BAD          ; reset PC
08  DCD loc_10400040+1      ; NMI handler
0C  DCD loc_10400042+1      ; Hard Fault handler
10  DCD loc_10400044+1      ; MPU Fault Handler
14  DCD loc_10400046+1      ; Bus Fault Handler
18  DCD loc_10400048+1      ; Usage Fault Handler
1C  DCD 0
20  DCD 0
24  DCD 0
28  DCD 0
2C  DCD loc_1040004A+1      ; SVCall Handler
30  DCD loc_1040004C+1      ; Debug Monitor Handler
34  DCD 0
38  DCD loc_1040004E+1      ; PendSV Handler
3C  DCD 0x10401645          ; SysTick Handler

40 loc_10400040
40   B loc_10400040
```

(The offsets have +1 because the code pointers on Cortex-M3 must have bit 0 set, to indicate that the target is Thumb code.)

The `loc_10400040` corresponds to the `B.N 0x40` in your disassembly. So, apparently, you ended up in the NMI handler.

Checking the LPC18xx User Manual, it seems NMI can only be triggered by the pins `P4_0` or `PE_4`. I'd suggest double-checking your board schematics and/or GPIO configuration code.

Software hanging at B.N instruction in disassembler -- No idea what it means

Solution

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