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Using Memory

To assign memory:

# store the value of esi in eax inside dword
mov dword [esi],eax

Constants and Data Initialization​

The syntax for storage allocation statement for initialized data is:

[variable-name]    define-directive    initial-value   [,initial-value]...

We can use constants in the following way:

entry start

...
SOME_CONSTANT = 6

...

; dd  == define doubleword (4 bytes)
; dup == duplicate
; (0) == initialize all 6 values to 0
section '.data' data readable writeable
    some_nums     dd SOME_CONSTANT dup (0)


sum_nums is a label stored in RAM where the data section starts. dd defines 6, duplicated double dwords which are all initialized to 0. So if the data section starts at memory address 400000, the double dwords will spread over 24 bytes starting from 400000 to 400024.

Initialized vs Uninitialized​

We use the .data section for initialized memory. For example, we're initializing 4001 dwords to 0:


; 4 because every dword is 4 bytes
; 100000h/ 4 + 1 = 4001
AMOUNT_NUMS = (10000h / 4) + 1


section `.data` data readable writable
 some_nums      dd AMOUNT_NUMS dup (0)

This would cause the program size to be large and contain many 0s. We can use the .bss section to save space:

AMOUNT_NUMS = (10000h / 4) + 1


section `.bss` readable writable
 some_nums      dd AMOUNT_NUMS dup (?) ;? don't want to initialize