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

User · Accepted Answer

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

User · Answer

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  quot h quot  instead of  quot 0x quot  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

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

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