Write X86/64 ALP to switch from real mode to protected mode and display the values of GDTR, LDTR, IDTR, TR and MSW Registers

Second Year Computer Engineering Microprocessor Programs:

Microprocessor Lab:

Practical 6:
Write X86/64 ALP to switch from real mode to protected mode and display the values of GDTR, LDTR, IDTR, TR and MSW Registers

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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,1 ;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
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