What value from memory or register do each of the AT&T syntax operands access?

Question

Assume the following values are stored at the indicated memory addresses and registers:

Address    Value            Register     Value
0x100      0xFF             %eax         0x100
0x104      0xAB             %ecx         0x1
0x108      0x13             %edx         0x3
0x10C      0x11


Fill in the following table showing the values for the indicated operands:

Operand           Value    //Solutions at the end of the chapter
%eax              _____    //0x100
0x104             _____    //0xAB
$0x108            _____    //0x108
(%eax)            _____    //0xFF
4(%eax)           _____    //0xAB
9(%eax, %edx)     _____    //0x11
260(%ecx, %edx)   _____    //0x13
0xFC(,%ecx,4)     _____    //0xFF
(%eax, %edx,4)    _____    //0x11

Can someone explain to me how to do this in layman's terms. This isn't hmwk (it's practice problems during certain readings which have the answers @ the end of the chapter), I'm just not understanding the reading.

score 13 · Accepted Answer · answered Feb 15 '13 at 07:32

13

The general rule for AT&T x86 assembly syntax is

displacement(offset, relative offset, multiplier) = offset + displacement + ( relative offset * multiplier)

%eax refers to actual value of the register(=0x100).
0x104 refers to the value at address 0x104.
$0x108 refers to the constant value 0x108.
(%eax) refers to the value at address EAX, which is equivalent to 0x100(=0xFF).
4(%eax) refers to the value at address EAX+4, which is at 0x104.
9(%eax, %edx) refers to the value at address EAX+9 + EDX, which is at 0x10C.
260(%ecx, %edx) refers to the value at address ECX+260 + EDX, which is at 0x108.
0xFC(,%ecx,4) refers to the value at address (ECX*4)+0xFC, which is at 0x100.
(%eax, %edx, 4) refers to the value at address (EAX+(EDX*4), which is at 0x10C.

answered Feb 15 '13 at 07:32

JosephH

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Much clearer... Still not understanding #7.. how can 1+ 4 + 260 = 0x108? – ShadyBears Feb 15 '13 at 07:35
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@CarlosCarrillo ecx is 1, edx is 3, so ECX+260+EDx=1+3+260=264=0x108 – JosephH Feb 15 '13 at 07:37
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O ok, I have to convert from decimal to hex. the other problems I don't, that's where I got confused. Thanks. – ShadyBears Feb 15 '13 at 07:40

score 6 · Answer 2 · answered Feb 15 '13 at 07:32

You need to learn this AT&T assembly syntax and a little bit of assembly and once you know that stuff, the answers are trivial.

So,

%eax is just the contents of the register.

0x104 is a memory operand, the contents of the memory at address 0x104.

$0x108 is a constant.

(some expression) is a memory operand, the contents of the memory at address some expression.

some constant(some expression) is a memory operand, the contents of the memory at address some expression + some constant.

(%register1, %register2) is, as you may have guessed by now, also a memory operand. The value of the parenthesized expression is register1 + register2.

some constant(%register1, %register2) should be trivial by now as well. Just add the three items and that's the address.

(, %register, some constant) means that you need to multiply the value of the register by the constant.

(%register1, %register2, some constant) should now be intuitive. The value of the parenthesized expression is register1 + register2 * some constant.

Guess now the address of the memory operand like this:

some constant1(%register1, %register2, some constant2).

What value from memory or register do each of the AT&T syntax operands access?

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