Assembly Tutorial (Continued)

As before we know the basic such as byte, nibble and bit. Today i'll tell you more about this basics.

1 WORD |0000000000000000|
2 BYTEs ÀÄ AH ÄÙÀÄ AL ÄÙ
4 NIBBLEs ÀÄÄÄÄÄ AX ÄÄÄÄÄÙ
16 BITs

The WORD is just two BYTEs that are stuck together. A word has a maximum value of 0FFFFh (= 65,535). Since a WORD is 4 NIBBLEs, it is represented by 4 HEX digits. This is the size of the 16bit registers on the 80x86 chips. The registers are: AX, BX, CX, DX, DI, SI, BP, SP, CS, DS, ES, SS, and IP. Note that you cannot directly change the contents of IP or CS in any way. They can only be changed by JMP, CALL, or RET.

1 DWORD
2 WORDs |00000000000000000000000000000000|
4 BYTEs ³ ÀÄ AH ÄÙÀÄ AL ÄÙ
8 NIBBLEs ³ ÀÄÄÄÄÄ AX ÄÄÄÄÄÙ
32 BITs ÀÄÄÄÄÄÄÄÄÄÄÄÄ EAX ÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

A DWORD (or "DOUBLE WORD") is just two WORDs, hence the name DOUBLE-WORD. This can have a maximum value of 0FFFFFFFFh (8 NIBBLEs, 8 'F's) which equals 4,294,967,295. Damn large. This is also the size or the 386's 32bit registers: EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP, EIP. The 'E ' denotes that they are EXTENDED registers. The lower 16bits is where the normal 16bit register of the same name is located.

1 KILOBYTE |-lots of zeros (8192 of 'em)-|
256 DWORDs
512 WORDs
1024 BYTEs
2048 NIBBLEs
8192 BITs

We've all heard the term KILOBYTE byte, before, so I'll just point out that a KILOBYTE, despite its name, is -NOT- 1000 BYTEs. It is actually 1024 bytes.

1 MEGABYTE |-even more zeros (8,388,608 of 'em)-|
1,024 KILOBYTEs
262,144 DWORDs
524,288 WORDs
1,048,576 BYTEs
2,097,152 NIBBLEs
8,388,608 BITs

Just like the KILOBYTE, the MEGABYTE is -NOT- 1 million bytes. It is actually 1024*1024 BYTEs, or 1,048,578 BYTEs

we will investigate an annoying little aspect of the 80x86 processors in later post. I'm talking about nothing other than SEGMENTS & OFFSETS!