As there are 24 PCR's in TPM 1.2 specification. Some of these PCRs are reserved and cannot be extended by user's code. Below are the PCR Index with their Usage

CRTM, BIOS and Platform Extensions Platform Configuration Option ROM Code Option ROM Configuration and Data IPL7Code (MBR Information and Bootloader Stage 1) IPL Code and Configuration Data (for use by IPL Code) State Transition and Wake Events Reserved for future usage. Do not use. Bootloader Stage 2 Part 1 Bootloader Stage 2 Part 2 Not in Use. Not in Use. Bootloader Command line Arguments Files checked via check-file routine Files which are actually loaded (e.g. Linux kernel, initrd, modules..) Not in Use. Not in Use. DRTM 18-23. Not in Use.

what I understood that a user can extend all the PCR's which are not in use? Is this correct? I asked this question Because I have written my own code to extend PCR's (by following https://www.cylab.cmu.edu/tiw/slides/challener-handout.pdf) and it turns out that i can extend all the PCR's except from PCR 17 to PCR 22. And my understanding was that I can only extend few and especially cannot play with the lower ones from PCR 0 to PCR 7.

Your help is highly appreciated! I can send the code as well if required!