Answer by Alexey Frunze for Basic use of immediates vs. square brackets in YASM/NASM x86 assembly

You are getting the idea. However, there are a few details worth bearing in mind:

1. Addresses can and usually are greater than what 8 bits can hold (cl is 8-bit, cx is 16-bit, ecx is 32-bit, rcx is 64-bit). So, cl is likely going to be unequal to the address of the variable buffer. It'll only have the least significant 8 bits of the address.
2. If there are interrupt routines or threads that can preempt the above code and/or access buffer, the value in bl may differ from 5. Broken interrupt routines may actually affect any register when they fail to preserve register values.