x86 16 - Is it possible to manipulate the instruction pointer in 8086 assembly?

Question

I want to know if I can manipulate (read and change the value of) the instruction pointer (IP) in 8086 assembly.

For example,

Say IP is currently storing 0200h. I would like to read this value and change it to something else, say 4020h. How could I do that?

Answer 1

If you wanted to set the instruction pointer to a known value, say hex value 4020h, you could jump directly to that address:

jmp 4020h

Or if some memory location, myVariable, held the value you wanted to store in IP you could do an indirect jump:

jmp [myVariable]

The result of a jmp (indirect or direct) modifies the instruction pointer.

Reading the instruction pointer is problematic. Position independent code on Linux used to work by using a set of code something like:

 call getIP

with

 :getIP
 mov bx, [sp] ; Read the return address into BX.
 ret

For other methods of reading IP, see Stack Overflow: reading IP.