Variable Addressing & Arrays
Every variable is just a location in memory at which a value is stored. Pointers will refer to the value of a memory location but sometimes we want it’s address.
Variable Addresses in Assembly
In assembly language we can get the address of a variable with the LEA (load effective address) instruction.
We often use the base register, ebx:
lea ebx, val
We can access the value pointed to by using the register indirect addressing mode:
mov eax, [ebx]
Using square brackets will treat the value as an address and fetch the value from that address.
Arrays
An array is just a sequence of values, stored consecutively in memory.
Array Storage
Different data types many occupy different amounts of memory.
If a grades[5] array is stored at memory address 1000 onwards, then:
- grades[0] is at 1000
- grades[1] is at 1004
- grades[2] is at 1008
- grades[3] is at 10016
Each value is 4 bytes as we are storing an int which is 4 bytes long in a 32 bit system.
We can use this to develop assembly code that accesses arrays.
Example
/* Sum the elements of an array. */
...
int my array[5]; // declaration of an array of integers
myarray[0] = 1; // initialise the array
myarray[1] = 3;
myarray[2] = 5;
myarray[3] = 7;
myarray[4] = 9;
_asm {
...
lea ebx, myarray // addr of array (0th element) in ebx
mov ecx, 5 // size of the array in ecx (for Loop1)
mov eax, 0 // initialise sum to 0
Loop1: add eax, [ebx] // get element pointed to by ebx
add ebx, 4 // to pint to next integer element
loop Loop1 // go round again
...
}
...