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

                                            Savitribai Phule Pune University
                            Second Year of Computer Engineering (2015 Course)
                                                     210257: Microprocessor Lab
                                                                Assignment No.06

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. 

;*****************************************************
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::"
gdtmsg_len equ $-gdtmsg

ldtmsg db 10,"LDT Contents are::"
ldtmsg_len equ $-ldtmsg

idtmsg db 10,"IDT Contents are::"
idtmsg_len equ $-idtmsg

trmsg db 10,"Task Register Contents are::"
trmsg_len equ $-trmsg

mswmsg db 10,"Machine Status Word::"
mswmsg_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

dispbuff resb 04

%macro display 2
    mov    rax,1    ;print
    mov    rdi,1    ;stdout/screen
    mov    rsi,%1    ;msg
    mov    rdx,%2    ;msg_len
    syscall
%endmacro

;*******************************************************
section .text
global _start
_start:
    smsw eax    ;Stores the machine status word (bits 0 
                ;through   15 of control register CR0) into eax.

    mov [cr0_data],eax

    bt eax,0    ;Checking PE(Protected Mode Enable) bit(LSB), 
                ;if 1=Protected Mode, else Real Mode
    jc prmode
    display rmodemsg,rmsg_len
    jmp nxt1

prmode: display pmodemsg,pmsg_len

nxt1:sgdt [gdt]    
     sldt [ldt]         
     sidt [idt]  
     str [tr]  
    ;.......display gdt data...........

     display gdtmsg,gdtmsg_len

     mov bx,[gdt+4]
     call display16_proc

     mov bx,[gdt+2]
     call display16_proc

     display colmsg,1

     mov bx,[gdt]
     call display16_proc
  
    ;......display ldt data...........
    
     display ldtmsg,ldtmsg_len
     mov bx,[ldt]
     call display16_proc

    ;......display idt data...........

    display idtmsg,idtmsg_len

    mov bx,[idt+4]
    call display16_proc

    mov bx,[idt+2]
    call display16_proc

    display colmsg,1

    mov bx,[idt]
    call display16_proc

    ;....display task register data..........

    display trmsg,trmsg_len

    mov bx,[tr]
    call display16_proc

    ;....display machine status word data......

    display mswmsg,mswmsg_len

    mov bx,[cr0_data+2]
    call display16_proc

    mov bx,[cr0_data]
    call display16_proc

    display nwline,1

    mov rax,60
    mov rdi,0
    syscall
;************************************************************
display16_proc:
    mov rdi,dispbuff    ;point esi to buffer
    mov rcx,4        ;load number of digits to display
dispup1:
    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 dispskip1        ;if less than 39h akip adding 07 more
    add dl,07h        ;else add 07

dispskip1:
    mov [rdi],dl        ;store ASCII code in buffer
    inc rdi            ;point to next byte
    loop dispup1        ;decrement the count of digits to display
                ;if not zero jump to repeat

    display dispbuff,4    ;
  
    ret
;******************Output*************************************
;[root@localhost MIT2016]# nasm -f elf64 proc.asm
;[root@localhost MIT2016]# ld -o proc proc.o
;[root@localhost MIT2016]# ./proc

;Processor is in Protected Mode
;GDT Contents are::B7304000:007F
;LDT Contents are::0000
;IDT Contents are::81BDD000:0FFF
;Task Register Contents are::0040
;Machine Status Word::8005FFFF
;[root@localhost MIT2016]#