ms_dos_segment.asm

; What does "BP.DEVNUM" mean?
DPBLOCK STRUC
DEVNUM  DB      ?       ;I/O driver number
DPBLOCK ENDS
MOV     AH,[BP.DEVNUM]         

;-------------------

; We see this pattern many times :
; SegmentRegister : [ PointerRegister.NAME ] 
; PointerRegister includes: BP , SP , DI , SI
; What does this pattern mean?
ADD     AX,ES:[BP.DIRSEC]

;-------------------

; Does " SEGBIOS SEGMENT AT BIOSSEG " mean 
; " The following declarations should be written at BIOSSEG" ?
; What does " ORG 0 " do?
; Line 175

BIOSSEG EQU     60H
SEGBIOS         SEGMENT AT BIOSSEG
                ORG     0
                DB      3 DUP (?)       ;Reserve room for jump to init code
BIOSSTAT        DB      3 DUP (?)       ;Console input status check
BIOSIN          DB      3 DUP (?)       ;Get console character
SEGBIOS ENDS


;-------------------

; On line 148, we see the following code
; This seems to imply the overall structure of the entire source
; But is it necessary?  Can the assembler write data first, and then code?
; ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓

;The following are all of the segments used
;They are declared in the order that they should be placed in the executable

CODE    SEGMENT
CODE    ENDS

CONSTANTS       SEGMENT BYTE
CONSTANTS       ENDS

DATA    SEGMENT WORD
DATA    ENDS

DOSGROUP        GROUP   CODE,CONSTANTS,DATA

SEGBIOS SEGMENT
SEGBIOS ENDS