Hi everyone I'm new to assembly and I am having a hard time to understand why the code below works. What I learnt is si holds a address of the data segment and because a and b sit in different places in the data segment it can't be used for both of them and I would have to use di for b. But for some reason the code below (which copies the first char of a to the first char of b) works fine
.model small
.stack 100H
.data
a db ' This is a test $'
b db 100 dup('$')
.code
mov si,0
mov al,a[si]
mov b[si],al
.exit
end
My main misunderstanding is what is the difference between a[si] to b[si] and to [si] any explanation would be highly appreciated.
What I learnt is si holds a address of the data segment
Traditionally. SI means "source index." However, ever since Protected Mode, the addressing modes have been loosened up a lot, so si can be used for a lot more. It's a general-purpose register, after all.
My main misunderstanding is what is the difference between a[si] to b[si] and to [si] [...]
[si] takes the address in si and references the data at that address.a[si] takes the address in si plus the address constant a to reference data.b[si] takes the address in si plus the address constant b to reference data.The line
mov al, a[si]
moves the byte at a + si into al.
mov b[si], al
moves the byte in al into the byte at b + si. That's a transitive chain of operations and so a[si] = b[si] after this.