ORG 7C00H

ENTRY:
    XOR BX,BX
    MOV DS,BX
    MOV SS,BX
    MOV ES,BX
    MOV BP,0FFFFH
    MOV SP,BP
    MOV [DriveNumber],DL
CHECKOSDISK:
    MOV SI,CODMessage0
    CALL WRITESTRING
    CALL RESETDRIVE
    MOV BX,513
    MOV CX,1
    MOV AL,1
    CALL READSECTOR
    JC CODERROR
    CMP BYTE [BX+508],42H
    JNE CODERROR
    CMP BYTE [BX+509],5AH
    JE CODNEXT
CODERROR:
    MOV SI,CODErrorMsg
    CALL WRITESTRING
    JMP ENDDELAY
CODNEXT:
    MOV SI,CODOKMessage
    CALL WRITESTRING
LOADOSMAIN:
    MOV SI,LOMessage
    CALL RESETDRIVE
    CALL WRITESTRING
    MOV BX,[OSMEntryOffset]
    MOV CX,2
    MOV AL,[OSMSectorCount]
    CALL READSECTOR
    JNC JUMPOSMAIN
LOMERROR:
    MOV SI,LOMErrorMsg
    CALL WRITESTRING
    JMP ENDDELAY
JUMPOSMAIN:
    MOV SP,BP
    JMP FAR [OSMEntryAddress]
ENDDELAY:
    MOV SI,EDMessage
    CALL WRITESTRING
    JMP $

WRITECHAR:                                ;AL = Character ASCII
    PUSH BX
    PUSH AX
    MOV BX,0FH
    MOV AH,0EH
    INT 10H
    POP AX
    POP BX
    RETN

WRITESTRING:                            ;DS:SI = Address Of String
    PUSH AX
WSLOOP:
    CLD
    LODSB
    TEST AL,AL
    JZ WSRET
    CALL WRITECHAR
    JMP WSLOOP
WSRET:
    POP AX
    RETN

WRITEHEXNUM:                            ;AX = Hex Number To Output
    PUSHAW
    CLD
    MOV CL,16
    MOV DX,AX
    MOV DI,HexNumTable
WHLOOP:
    MOV AX,DX
    MOV SI,DI
    SUB CL,4
    JZ WHNEXT
    SHR AX,CL
WHNEXT:
    AND AX,0000000000001111B
    ADD SI,AX
    LODSB
    CALL WRITECHAR
    TEST CL,CL
    JNZ WHLOOP
    POPAW
    RETN

WRAPTEXT:
    MOV AL,0DH
    CALL WRITECHAR
    MOV AL,0AH
    CALL WRITECHAR
    RETN

RESETDRIVE:                                ;DL = Drive Number
    XOR AH,AH
    INT 13H
    RETN                                ;Succeed: CF = 0, AH = 0; Failed: CF = 1, AH = Error Code

READSECTOR:                                ;AL = Number Of Sectors To Read, CH = Low 8 Bits Of Cylinder Number, CL = Sector Number(Bit 0-5) High 2 Bits Of Cylinder Number, DH = Head Number, DL = Drive Number, ES:BX = Address Of Buffer
    PUSH BP
    PUSH 2
    MOV BP,SP
RSLOOP:
    PUSHAW
    MOV AH,2
    INT 13H
    JNC RSRET
RSRETRY:
    DEC WORD [BP]
    POPAW
    JNZ RSLOOP
RSRET:
    ADD SP,18
    POP BP
    RETN                                ;Succeed: CF = 0, AH = 0, AL = Number Of Sectors Transferred; Failed: CF = 1, AH = Error Code

WRITESECTOR:                            ;AL = Number Of Sectors To Write, CH = Low 8 Bits Of Cylinder Number, CL = Sector Number(Bit 0-5) High 2 Bits Of Cylinder Number, DH = Head Number, DL = Drive Number, ES:BX = Address Of Buffer
    PUSH BP
    PUSH 2
    MOV BP,SP
WSELOOP:
    PUSHAW
    MOV AH,3
    INT 13H
    JNC WSERET
WSERETRY:
    DEC WORD [BP]
    POPAW
    JNZ WSELOOP
WSERET:
    ADD SP,18
    POP BP
    RETN                                ;Succeed: CF = 0, AH = 0, AL = Number Of Sectors Transferred; Failed: CF = 1, AH = Error Code

DriveInfo:
DriveNumber:
    DB 0
MaxCylinderNum:
    DW 0
MaxSectorNum:
    DB 0
MaxHeadNum:
    DB 0

OSMainInfo:
OSMEntryOffset:
    DW 7E00H
OSMSectorCount:
    DB 4
OSMEntryAddress:
    DD 07E00000H
   

Tables:
CharTable:
    DB "ABCDEFGHIJKLMNOPQRSTUVWXYZ",0
HexNumTable:
    DB "0123456789ABCDEF",0

Strings:
EDMessage:
    DB "SYSTEM PENDING...",0DH,0AH,0
CODMessage0:
    DB "CHECKING OS DRIVE...",0DH,0AH,0
CODOKMessage:
    DB "OS DRIVE IS NORMAL.",0DH,0AH,0
LOMessage:
    DB "LOADING OS MAIN, SYSTEM KERNEL...",0DH,0AH,0
CODErrorMsg:
    DB "ERROR: CURRENT DRIVE UNAVAILABLE.",0DH,0AH,0
LOMErrorMsg:
    DB "ERROR: OS MAIN LOAD FAILED.",0DH,0AH,0


BootSectSignature:
    TIMES 508-($-$$) DB 0
    DB 42H,5AH
    DB 55H,0AAH