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Editorial Team
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Asked: 2026-05-17T22:50:29+00:00

I have started to learn assembly. I came across these lines. ;*************************************************; ; OEM

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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.

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1 Answer

  1. Editorial Team

    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
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