WinDbg for 32 bits and 64 bits has a set of internal pseudo-registers that you can use as variables or as a means to get specific information.
The pseudo-registers are, according to WinDbg documentation:
The effective address of the last instruction that was executed. If this instruction does not have an effective address, the debugger displays "Bad register error.” If this instruction has two effective addresses, the debugger displays the first address.
The second effective address of the last instruction that was executed. If this instruction does not have two effective addresses, the debugger displays "Bad register error.”
The last expression that was evaluated.
The return address that is currently on the stack.
The instruction pointer register.
The instruction pointer at the time of the current event. This pointer typically matches $ip, unless you switched threads or manually changed the value of the instruction pointer.
The instruction pointer at the time of the previous event. (Breaking into the debugger counts as an event.)
An instruction pointer that is related to the current event. When you are branch tracing, this pointer is the pointer to the branch source.
The instruction pointer for the current local context (also known as the scope).
The address of the entry point of the first executable of the current process.
The primary return value register.
The primary return value register, in 64-bit format.
The current call stack pointer. This pointer is the register that is most representative of call stack depth.
The value that the last d* (Display Memory) command printed.
The address of the current process (that is, the address of the EPROCESS block).
The address of the current thread (that is, the address of the ETHREAD block).
The address of the process environment block (PEB) of the current process.
The address of the thread environment block (TEB) of the current thread.
The process ID (PID) for the process that owns the current thread.
The thread ID for the current thread.
The address of the corresponding breakpoint. For example, $bp3 (or $bp03) refers to the breakpoint which has a breakpoint ID of 3. Number is always a decimal number. If no breakpoint has an ID of Number, $bpNumber evaluates to zero.
The current frame index. This index is the same frame number that the .frame (Set Local Context) command uses.
The current time, according to the computer that the debugger is running on.
The return value of the last function that .call (Call Function) called or that is used in an .fnret /s command. The data type of $callret is the data type of this return value.
The size of a pointer. In kernel mode, this size is the pointer size on the target computer.
The number of bytes in one page of memory. In kernel mode, this size is the page size on the target computer
Besides we have User-Defined Pseudo-Registers:
There are 20 user-defined pseudo-registers ($t0, $t1, ..., $t19). These pseudo-registers are variables that you can read and write through the debugger. You can store any integer value in these pseudo-registers. They can be especially useful as loop variables.
To write to one of these pseudo-registers, use the r (Registers) command, as the following example shows:
r @$t0 = 7
r @$t1 = 128 * poi(@$t0)
Like all pseudo-registers, you can use the user-defined pseudo-register in any expression, as the following example shows:
?? @$t1 + 4 * @$t2
Tip: A pseudo-register is always typed as an integer, unless you use the ? switch together with the r command. If you use this switch, the pseudo-register acquires the type of whatever is assigned to it. For example, the following command assigns the UNICODE_STRING** type and the 0x0012FFBC value to $t15.
r? @$t15 = * (UNICODE_STRING*) 0x12ffbc
User-defined pseudo-registers use zero as the default value when the debugger is started.
Tip: To improve performance always use @ before the register or pseudo-register. In other words:
r @$t0 = @$t0 + 0n10 is faster than r $t0 + $t0 + 0n10.
r @ecx = @eax + 0x10 is faster than r ecx = eax + 0x10
Tip: To assign a new value to a register or pseudo-register or to do a mathematical operation, use the r command. Also, to display a register/pseudo-register value, the r command is cleaner than ?. Use ?? when you want to know the type.
See the difference:
It’s very common to use registers or pseudo-registers with the poi command.
poi dereferences an address. Think about * in C and C++.
Pseudo-registers are heavily used in breakpoints and scripts; thus, they are an important asset to dominate.
For more, read these articles.