The following GNU Assembly example for x86 is taken from the book "Programming from the Ground Up":
1 #VARIABLES: The registers have the following uses:
2 #
3 # %edi - Holds the index of the data item being examined
4 # %ebx - Largest data item found
5 # %eax - Current data item
6 #
7 # The following memory locations are used:
8 #
9 # data_items - contains the item data. A 0 is used
10 # to terminate the data
11
12 .section .data
13
14 data_items:
15 .long 3,67,34,222,45,75,54,34,44,33,22,11,66,0
16
17 .section .text
18
19 .globl _start
20
21 _start:
22 movl $0, %edi #move a 0 into the edi register
23 movl data_items(,%edi,4), %eax #load the first byte of data
24 movl %eax, %ebx
I'm struggling to understand the syntax of line 23. Looking at the syntax I can deduce the following:
- movl command moves the list contents in the location designated by data_items to register eax.
But I'm not sure how to exactly understand the notation "(,%edi,4)", i.e:
- why is there an empty argument there in the list of arguments?
- what is the 4 for?
How would I read out in human language line 23 so that it makes consistent sense?
