Debug Command 29 - Exchange Opcode and Go Command

Parameters

Pid

= Process ID of debuggee
Tid
= Thread ID of thread
Cmd
= DBG_C_XchngOpcode
Value
= Opcode 1 for Single Step
Addr
= Opcode 2 for Go

Returns

The sequence of operations for this Debug command is:

• Replace the code at the EIP (instruction pointer) with opcode 1.

• Single-step the thread specified by the Tid field. Do not execute other threads. If the single-step operation goes into ring 0 code, consider the single-step operation complete at the first ring 0 instruction.

• Replace the code at the original EIP with opcode 2.

• Issue a Debug Go command on all non-frozen threads.

If an exception that DosDebug is to be notified about occurs during the single-step operation of this Debug command, opcode 2 is placed at the original EIP, and DosDebug is notified of the exception. When the debugger issues the Debug Continue command, the child process continues execution.

Note: If an exception that DosDebug is not to be notified about occurs, then the DBG_C_XchngOpcode command executes as if no exception took place.

If opcode 1 and opcode 2 are identical, this Debug command executes only operations 3 and 4 above. There is no need to single-step the thread specified by the Tid field. This would be a "replace opcode and go" sequence using only one DosDebug function instead of two.

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