; THIS CODE HAS NOT BEEN TESTED AND IS NOT FULLY OPERATIONAL.

; Problem Statement: Write X86/64 ALP to switch from real mode to protected mode and display the values of GDTR, LDTR, IDTR, TR and MSW Registers also identify CPU type using CPUID instruction.

; Code from Microprocessor (SPPU - Second Year - Computer Engineering - Content) repository on KSKA Git: https://git.kska.io/sppu-se-comp-content/Microprocessor/


; BEGINNING OF CODE
section .data
	rmodemsg db 10,'Processor is in REAL MODE.',
	rmsg_len:equ $-rmodemsg

	pmodemsg db 10,'Processor is in PROTECTED MODE.'
	pmsg_len:equ $-pmodemsg

	gdtmsg db 10,'GDT Contents are: '
	gmsg_len:equ $-gdtmsg

	ldtmsg db 10,'LDT Contents are: '
	lmsg_len:equ $-ldtmsg

	idtmsg db 10,'IDT Contents are: '
	imsg_len:equ $-idtmsg

	trmsg db 10,'Task Register Contents are: '
	tmsg_len: equ $-trmsg

	mswmsg db 10,'Machine Status Word: '
	mmsg_len:equ $-mswmsg

	colmsg db ':'

	nwline db 10

section .bss
	gdt resd 1
	    resw 1
	ldt resw 1
	idt resd 1
	    resw 1
	tr  resw 1

	cr0_data resd 1

	dnum_buff resb 04

%macro print 2
	mov rax,01
	mov rdi,01
	mov rsi,%1
	mov rdx,%2
	syscall
%endmacro

section .text
global _start
_start:	
	smsw eax							;Reading CR0. As MSW is 32-bit cannot use RAX register. 

	mov [cr0_data],rax

	bt rax,0							;Checking PE bit, if 1=Protected Mode, else Real Mode
	jc prmode
	print rmodemsg,rmsg_len
	jmp nxt1

prmode:	print pmodemsg,pmsg_len

nxt1:sgdt [gdt]
	sldt [ldt]
	sidt [idt]
	str [tr]
	print gdtmsg,gmsg_len
	
	mov bx,[gdt+4]
	call print_num

	mov bx,[gdt+2]
	call print_num

	print colmsg,1

	mov bx,[gdt]
	call print_num

	print ldtmsg,lmsg_len
	mov bx,[ldt]
	call print_num

	print idtmsg,imsg_len
	
	mov bx,[idt+4]
	call print_num

	mov bx,[idt+2]
	call print_num

	print colmsg,1

	mov bx,[idt]
	call print_num

	print trmsg,tmsg_len
	
	mov bx,[tr]
	call print_num

	print mswmsg,mmsg_len
	
	mov bx,[cr0_data+2]
	call print_num

	mov bx,[cr0_data]
	call print_num

	print nwline,1


exit:	mov rax,60
	xor rdi,rdi
	syscall

print_num:
	mov rsi,dnum_buff			;point esi to buffer

	mov rcx,04						;load number of digits to printlay 

up1:
	rol bx,4							;rotate number left by four bits
	mov dl,bl							;move lower byte in dl
	and dl,0fh						;mask upper digit of byte in dl
	add dl,30h						;add 30h to calculate ASCII code
	cmp dl,39h						;compare with 39h
	jbe skip1							;if less than 39h skip adding 07 more 
	add dl,07h						;else add 07
skip1:
	mov [rsi],dl					;store ASCII code in buffer
	inc rsi								;point to next byte
	loop up1							;decrement the count of digits to printlay
												;if not zero jump to repeat

	print dnum_buff,4			;printlay the number from buffer
	
	ret
; END OF CODE