op code explaination required

[shahzad]

according to listing file generated by nasm the op code for instruction

Code: Select all

mov [120h],bx

is

Code: Select all

891E2001

89 is for moving r16 to r/m16 (r is register and m is memory)

2001 is memory location

can you please give description of middle byte 1E

[Jamethiel]

It's a mod/rm byte. In the case of 1E, it specifies a reg of bx and an r/m of [offs16].

Now do me a favor and hit up Intel's website for the pdfs of the Pentium manuals? They cover this sort of stuff in far more detail than I'm prepared to.