Which variable size to use (db, dw, dd) with x86 assembly?

Question

I am a beginner to assembly and I don't know what all the db, dw, dd, things mean. I have tried to write this little script that does 1+1, stores it in a variable and then displays the result. Here is my code so far:

.386 .model flat, stdcall  option casemap :none include \masm32\include\windows.inc include \masm32\include\kernel32.inc include \masm32\include\masm32.inc includelib \masm32\lib\kernel32.lib includelib \masm32\lib\masm32.lib .data num db ? ; set variable . Here is where I don't know what data type to use. .code start: mov eax, 1               ; add 1 to eax register mov ebx, 1               ; add 1 to ebx register add eax, ebx             ; add registers eax and ebx push eax                 ; push eax into the stack pop num                  ; pop eax into the variable num (when I tried it, it gave me an error, i think  thats because of the data type) invoke StdOut, addr num  ; display num on the console. invoke ExitProcess       ; exit end start 

I need to understand what the db, dw, dd things mean and how they affect variable setting and combining and that sort of thing.

Thanks in advance, Progrmr

Answer 1

Quick review,

• DB - Define Byte. 8 bits
• DW - Define Word. Generally 2 bytes on a typical x86 32-bit system
• DD - Define double word. Generally 4 bytes on a typical x86 32-bit system

From x86 assembly tutorial,

The pop instruction removes the 4-byte data element from the top of the hardware-supported stack into the specified operand (i.e. register or memory location). It first moves the 4 bytes located at memory location [SP] into the specified register or memory location, and then increments SP by 4.

Your num is 1 byte. Try declaring it with DD so that it becomes 4 bytes and matches with pop semantics.

Answer 2

The full list is:

DB, DW, DD, DQ, DT, DDQ, and DO (used to declare initialized data in the output file.)

See: http://www.tortall.net/projects/yasm/manual/html/nasm-pseudop.html

They can be invoked in a wide range of ways: (Note: for Visual-Studio - use "h" instead of "0x" syntax - eg: not 0x55 but 55h instead):

    db      0x55                ; just the byte 0x55     db      0x55,0x56,0x57      ; three bytes in succession     db      'a',0x55            ; character constants are OK     db      'hello',13,10,'$'   ; so are string constants     dw      0x1234              ; 0x34 0x12     dw      'A'                 ; 0x41 0x00 (it's just a number)     dw      'AB'                ; 0x41 0x42 (character constant)     dw      'ABC'               ; 0x41 0x42 0x43 0x00 (string)     dd      0x12345678          ; 0x78 0x56 0x34 0x12     dq      0x1122334455667788  ; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11     ddq     0x112233445566778899aabbccddeeff00     ; 0x00 0xff 0xee 0xdd 0xcc 0xbb 0xaa 0x99     ; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11     do      0x112233445566778899aabbccddeeff00 ; same as previous     dd      1.234567e20         ; floating-point constant     dq      1.234567e20         ; double-precision float     dt      1.234567e20         ; extended-precision float 

DT does not accept numeric constants as operands, and DDQ does not accept float constants as operands. Any size larger than DD does not accept strings as operands.