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What is the maximum number of sectors I can read with 2h function?

Is there any limitation apart from the AL register (255 max) and the amount of memory?

Thanks...

These sorts of questions are exactly what Ralf Brown's Interrupt List (RBIL) is supposed to help with.
Unfortunately, the answer is BIOS-specific. I haven't looked at my copy of RBIL, but I'd expect that you had best keep your transfers below 64K, and probably <128 sectors (NOT including 128). I have heard that some BIOSes have problems with transfers of 128 sectors, for a different Int13h command -- and I would expect it would happen for all Int13h commands on that BIOS. I always use max 32K transfers, myself.

I'm sorry if I seemed to be lazy, I didn't know RBIL list of interrupts could help me. Anyway, thanks for your reply.

with sub function 0x02h there is a maximum of 255 sectors per call, use subfunction 0x42h if you want to read more per call, as its limit is 65535 per call

Not really. The extended services (AH=42H) only allow you to read 127 sectors at a time.