Explanation of assembly code

Question

I have started to learn assembly. I came across these lines.

;*************************************************;
;   OEM Parameter block / BIOS Parameter Block
;*************************************************;

TIMES 0Bh-$+start DB 0

bpbBytesPerSector:      DW 512
bpbSectorsPerCluster:   DB 1
bpbReservedSectors:     DW 1

I am confused on the first line of declaration "bpbBytesPerSector: DW 512" . Here I think DW is define word. So DW 512 means defining 512 words means 1024 bytes. Now the label is "bpbBytesPerSector". Bytes per sector should be 512(this is what I think). Similarly I can't understand next two lines. I am totally confused on this. Can anybody explain me.Thanks in advance.

score 6 · Accepted Answer · answered Nov 05 '10 at 06:21

No, dw means define the single word 512. That means allocate space for one word here and set the value to 512.

This is creating a BIOS parameter block (BPB) and breaking it down:

0000                         TIMES 0Bh-$+start DB 0     ; allocate 11 zero bytes.
000B  bpbBytesPerSector:     DW 512                     ; define one word 512
000D  bpbSectorsPerCluster:  DB 1                       ; define one byte 1
000E  bpbReservedSectors:    DW 1                       ; define one word 1
0010

score 0 · Answer 2 · answered Nov 05 '10 at 18:06

As @paxdiablo already pointed out, the number is just the value for the word. When/if you want to define an array of 512 words like you described, you'd use something like:

myarray dw 512 dup(?)

Here the 512 is the number of repetitions, and the ? is the value to be put in each ("?" means "leave it uninitialized", but you can specify a value if you prefer).

Explanation of assembly code

2 Answers2