MAIN.INC

%INCLUDE "FAT32HEADERS_ADDR.INC"
READ_ATA:
	;ACCESS DISK VIA INT 0X13

	;SAVE THE VALUE
	PUSHAD

	;THE DISK PACKET WILL BE LOCATED AT 0X500 (RIGHT AFTER EBDR DATA REGION)
	%DEFINE PACKETBASE 0X500
	;WORD 0X00 (RESERVED)
	;PACKET SIZE (0X10)
	;0X00, 0X10 -> 0X1000 IN LSB
	%DEFINE PACKET_HEADER 0X1000
	;HOW MANY SECTORS TO READ
	%DEFINE SECTOR_COUNT (2)
	;LINEAR ADDRESS WILL BE BREAK UP INTO SEGMENT:OFFSET FORM
	%DEFINE BUFFER_OFFSET (4)
	%DEFINE BUFFER_SEG (6)
	;LOGIC BLOCK ADDRESS OFFSET (HIGH, LOW)
	%DEFINE LOW_LBA_NUMBER (8)
	%DEFINE HIGH_LBA_NUMBER (12)
	
	.SETUP_PACKET:
	;CREATE THE PACKET STRUCTURE
	;USE AX AS INDEX IN ORDER TO SHRINK THE SIZE OF THE PROGRAM
	MOV EBP, EBX
	MOV BX, PACKETBASE

	MOV WORD [BX], PACKET_HEADER
	MOV DWORD [BX+HIGH_LBA_NUMBER], 0

	;CONVERT LINEAR ADDRESS INTO SEGMENT AND OFFSET
	MOV WORD [BX+BUFFER_OFFSET],BP
	SHR EBP, 16
	MOV WORD [BX+BUFFER_SEG],BP

	MOV WORD [BX+SECTOR_COUNT],CX
	
	;BX=HIGHER PART OF THE ADDRESS(SEGMENT NUMBER)
	;THE EXTENDED REGISTER IS ONLY ABLE TO ADDRESS 32 BIT LBA
	
	MOV DWORD [BX+LOW_LBA_NUMBER], EAX
	MOV DWORD [BX+HIGH_LBA_NUMBER], 0
	
	;ISSUE AN 0X13 INTERRUPT
	MOV AH, 0X42
	;MOV DL, 0X80	;DRIVE NUMBER
	;LET'S NOT HARD CODED THE DRIVE NUMBER INTO THE PROGRAM
	MOV DL, BYTE [BS_DRVNUM]
	MOV SI, PACKETBASE
	INT 0X13
	;IF THE CARRY FLAG IS SET, THERE MUST BE AN ERROR, HALT
	;JC $

	POPAD
	RET


NXT_CLUSTER:
	
	
	;CLUSTER NUMBER STARTS FROM 2
	;THAT IS TO SAY, THE FIRST DATA SECTOR IS CLUSTER 2, AND SO ON.
	;BUT IN THIS FUNCTION, ASSUME CLUSTER NUMBER STARTS FROM 0 (THE FIRST DATA SECTOR IS CLUSTER 0)

	;STEP 1: IF THE FIRST DATA SECTOR IS CLUSTER 0, THAN PLUS 2
	;ADD EAX, DWORD [BPB_ROOTCLUS]

	;STEP 2: LOCATE THE NXTCLUS ENTRY IN FAT TABLE 
	;NOW, THE EAX REGISTER IS THE CLUSTER NUMBER (STARTS FROM 2)

	;STEP 2.1: MUL EAX BY 4 ((OPERATOR *4) = (OPERATOR << 2))
	SHL EAX, 2

	;STEP 2.2: EAX = EAX + FATBASE
	ADD EAX, FATBASE

	;STEP 3:EXTRACT THE VALUE OF "NXTCLUSTER", WRITEBACK INTO EAX
	MOV EAX, DWORD [EAX]

	;SUB EAX, DWORD [BPB_ROOTCLUS]

	RET


CLUSTER2SECTOR:
	PUSH ECX
	PUSH EDX
	

	XOR EDX, EDX
	;EAX IS PASSED BY THE CALLER TO CALLEE
	MOVZX ECX, BYTE [BPB_SECPERCLUS]
	MUL CX
	ADD EAX, DWORD [FIRSTDATASECTOR]

	POP EDX
	POP ECX
	RET


CMP_STOP:
	;IS IT ORDINARY CLUSTER?
	CMP EAX, 0X00_00_00_02
	JGE .IN_VALID_CLUSTERNUM_RANGE
	.IN_VALID_CLUSTERNUM_RANGE:CMP EAX, 0X0F_FF_FF_EF
	JBE .RETURN_LABEL	;FINISH READING, RETURN

	;HAVE IT REACHED EOF?
	.NOT_VALID_COMPARE_EOF: CMP EAX, 0X0F_FF_FF_F8
	JGE .IN_EOF_VALUE_RANGE
	.IN_EOF_VALUE_RANGE: CMP EAX, 0X0F_FF_FF_FF
	JNBE .RETURN_LABEL

	;IF (EOF)
	;JMP BP
	PUSH BP
	
	.RETURN_LABEL:
		RET