ARM ARM7TDMI - Coupling Breakpoints and Watchpoints

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Debug in Depth

B.18 Coupling breakpoints and watchpoints

You can couple watchpoint units 1 and 0 together using the CHAIN and RANGE

inputs. Using CHAIN enables Watchpoint 0 to be triggered only if Watchpoint 1 has

previously matched. The use of RANGE enables simple range checking to be

performed by combining the outputs of both watchpoints.

B.18.1 Breakpoint and watchpoint coupling example

Let:

Av[31:0]

be the value in the address value register

Am[31:0]

be the value in the address mask register

A[31:0]

be the address bus from the ARM7TDMI core

Dv[31:0]

be the value in the data value register

Dm[31:0]

be the value in the data mask register

D[31:0]

be the data bus from the ARM7TDMI core

Cv[8:0]

be the value in the control value register

Cm[7:0]

be the value in the control mask register

C[9:0]

be the combined control bus from the ARM7TDMI core, other

watchpoint registers and the EXTERN signal.

CHAINOUT signal

The CHAINOUT signal is derived as follows:

WHEN (({Av[31:0],Cv[4:0]} XNOR {A[31:0],C[4:0]}) OR {Am[31:0],Cm[4:0]} == 0xFFFFFFFFF)

CHAINOUT = ((({Dv[31:0],Cv[7:5]} XNOR {D[31:0],C[7:5]}) OR {Dm[31:0],Cm[7:5]}) == 0x7FFFFFFFF)

The CHAINOUT output of watchpoint register 1 provides the CHAIN input to

Watchpoint 0. This CHAIN input enables you to specify complicated configurations of

breakpoints and watchpoints.

Note

There is no CHAIN input to Watchpoint 1 and no CHAIN output from Watchpoint 0.

Consider, for example, the request by a debugger to breakpoint on the instruction at

location YYY when running process XXX in a multi-process system. If the current

process ID is stored in memory, you can implement the above function with a

watchpoint and breakpoint chained together. The watchpoint address points to a known

memory location containing the current process ID. The watchpoint data points to the

required process ID. The ENABLE bit is cleared.

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