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what does movsbl instruction do?

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Upon disassembling a program, I found a really peculiar instruction:

0x0000000000401106: movsbl (%rbx,%rax,1),%ecx

I think I know what movsbl does: It basically extends a byte and adds leading ones (sign extended) to the register.

But I have absolutely no clue what it does when it is used in the above context.

Any light to shed on this instruction would be most appreciated!

Answer 1

The instruction

movsbl <%rbx,%rax,1>,%ecx

reads one byte from the memory location addressed by the first operand, sign extends the byte to 32 bits, and stores the result in the ecx register.

Now to <%rbx,%rax,1>. This simply denotes the memory address formed by adding together the values of rbx and rax. In case you’re wondering, the 1 is the multiplier applied to rax.