Addressing Modes in Assembly Language (IA-32 NASM)
addressing-mode, assembly, nasm, x86
Solution
In NASM syntax, that instruction should be `MOV EBX, MY_TABLE`. What `MOV EBX, [MY_TABLE]` would do is load the first 4 bytes located at `MY_TABLE` into `EBX`. Another alternative would be to use `LEA`, as in `LEA EBX, [MY_TABLE]`.
In this case the tutorial is right. `MY_TABLE` is defined as an array of words. A word on the x86 is 2 bytes, so the second element of `MY_TABLE` is indeed located at `MY_TABLE + 2`.
Problem
As the web-resources on this is sparse, I will, for the benefit of future searches, begin by listing the address modes for IA-32 Assembly Language (NASM) and then follow up with a quick question. - Register addressing - mov eax, ebx: Copies what is in ebx into eax - mov esi, var: Copies address of var (say 0x0040120e) into esi - Immediate addressing (second operand is an immediate constant) - mov bx, 20: 16-bit register bx gets the actual value 20 - Direct memory addressing (directly loads from memory through a specified address) - mov ax, [1000h]: loads a 2-byte object from the byte at address 4096 (0x1000 in hexadecimal) into a 16-bit register called 'ax' - mov [1000h], ax: memory at address 1000h gets the value of ax - Direct offset addressing (same as 3, just using arithmetics to modify address) - mov al, [byte_tbl+2] - Register indirect (accessing memory by using addresses stored in registers) - mov ax, [di]: copies value at memory address specified by di, into ax - mov dword [eax], var1: copies value in var1 into the memory slot specified by eax Please note that the above is for NASM. For MASM/TASM you'd use "mov esi, OFFSET foo" to get the address, while "mov esi, foo" and "mov esi, [foo]" both would get the value (creds to @Michael). So, onto my question. It is in in relation to an example at the bottom of page 29 of the following tutorial: http://www.tutorialspoint.com/assembly_programming/assembly_tutorial.pdf It basically lists the below code as an example of indirect memory addressing. ``` MY_TABLE TIMES 10 DW 0 ; Allocates 10 words (2 bytes) each initialized to 0 MOV EBX, [MY_TABLE] ; Effective Address of MY_TABLE in EBX MOV [EBX], 110 ; MY_TABLE[0] = 110 ADD EBX, 2 ; EBX = EBX +2 MOV [EBX], 123 ; MY_TABLE[1] = 123 ``` My questions: - Should not "MOV EBX, [MY_TABLE]" in fact be "MOV EBX, MY_TABLE", as we want to put the address of the table in EBX, not the value itself? - Surely it is MY_TABLE[2] that is equal to 123 at the end, not MY_TABLE[1]?