View source for DBG C SStep

Debug Command 9 - Single Step Command 

;Parameters 

*Pid= Process ID of debuggee 
*Tid= Thread ID of thread to single-step 
*Cmd= DBG_C_SStep 

If Tid is zero, all threads will be marked to single-step at once, and the first thread to be scheduled to execute user-space code will single-step. No other threads will single-step. 

;Returns 

Usually, the DBG_N_Exception notification is returned, but any notification may be returned. See DosDebug Notifications for more information. 

Callgates that result in a privilege level transition to ring 0 will appear to single-step as a single instruction, with the single-step occurring just after the function completes. This hides ring 0 execution from debuggers. 

Attempting to single-step any thread that is frozen results in an error. 

;Restrictions 

The DBG_C_SStep command has two modes of operation, as follows: 

*If Tid is zero, the current thread is single-stepped while allowing all other threads to execute. 
*If Tid is nonzero, a specific thread is selected for single-stepping. Only that thread is executed, even if it is single-stepping a kernel function that can potentially cause a deadlock condition. 

The single-step exception (XCPT_SINGLE_STEP) is not lost if the single-step operation causes a notification to be sent to DosDebug. In this case, the single-step exception is queued. 

The single-step operation is not lost if other notifications were queued before the DBG_C_SStep command was issued. The Debug DBG_C_Continue command will clear the notifications one at a time until DosDebug has been completely notified. On the last DBG_C_Continue command, the single-step operation will take place as originally requested. 

When a single-step operation is interrupted by an exception, the EIP (instruction pointer) should be moved to the next RING3 instruction. This may be in ring 3 system code. The single-step notification will be issued at this time. 

The DBG_C_SStep command correctly single-steps most instructions. Single-stepping some REP instructions may not work correctly due to errors in the 80386 processor. 

[[Category:DBG_C]]
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