How to efficiently debug Loops with OllyDbg/Immunity?

Question

I'm currently reversing a function which looks like the following

.text:0040383F 8D 04 BF          lea     eax, [edi+edi*4]
.text:00403842 6A 14             push    20
.text:00403844 C1 E0 03          shl     eax, 3
.text:00403847 99                cdq
.text:00403848 59                pop     ecx
.text:00403849 F7 F9             idiv    ecx
.text:0040384B 03 45 08          add     eax, [ebp+arg_0]
.text:0040384E 8A 84 30 C8 31 00+mov     al, [eax+esi+31C8h]
.text:00403855 32 C3             xor     al, bl
.text:00403857 88 84 3E 28 27 00+mov     [esi+edi+2728h], al
.text:0040385E 47                inc     edi
.text:0040385F 81 FF 07 0B 00 00 cmp     edi, 0B07h
.text:00403865 75 D8             jnz     short loc_40

Since I don't have any clue what's going there I wanted to Debug this part with OllyDbg. I want to understand what's inside al, bl and the result of xor al, bl for all "0B07h" steps the loop is running.

I just saw that Immunity provides some sort of scripting functionality. Is it possible to achieve this with a simple python script in Immunity? Maybe there are other ways with OllyDbg?

I just want something like:

If EIP == "403855" then print al, bl
Else go_ahead

Answer 1

No scripting required.

In OllyDbg's disassembly window, left-click on line .text:00403855 32 C3 xor al, bl to select the line, then right-click on the selected line and choose Breakpoint → Conditional log....

In the breakpoint dialog box that opens up, use the following options:

Press OK, run the program, and every time .text:00403855 32 C3 xor al, bl is executed, OllyDbg will print the values of al and bl to the log window.