ASS 5

;-------------------------------------------------------------------
section    .data
        nline        db        10,10
        nline_len        equ        $-nline
        colon                db        ":"


        rmsg         db         10,'Processor is in Real Mode...'
        rmsg_len        equ         $-rmsg


        pmsg         db         10,'Processor is in Protected Mode...'
        pmsg_len        equ         $-pmsg


        gmsg                db        10,"GDTR (Global Descriptor Table Register)        : "
        gmsg_len        equ        $-gmsg


        imsg                db        10,"IDTR (Interrupt Descriptor Table Register)        : "
        imsg_len        equ        $-imsg


        lmsg                db        10,"LDTR (Local Descriptor Table Register)        : "
        lmsg_len        equ        $-lmsg


        tmsg                db        10,"TR (Task Register)                : "
        tmsg_len        equ        $-tmsg


        mmsg                db        10,"MSW (Machine Status Word)        : "
        mmsg_len        equ        $-mmsg
;---------------------------------------------------------------------
Section   .bss
        GDTR                resw        3                ; 48 bits, so 3 words
        IDTR                resw        3
        LDTR                resw        1                ; 16 bits, so 1 word
        TR                resw        1
        MSW                resw        1


        char_ans        resb        4                ; 16-bits, so 4 digits
;---------------------------------------------------------------------
%macro  Print   2
        mov   rax, 1
        mov   rdi, 1
        mov   rsi, %1
        mov   rdx, %2
        syscall
%endmacro


%macro  Read   2
        mov   rax, 0
        mov   rdi, 0
        mov   rsi, %1
        mov   rdx, %2
        syscall
%endmacro


%macro        Exit        0
        mov  rax, 60
        mov  rdi, 0
        syscall
%endmacro


;---------------------------------------------------------------------


section    .text
        global   _start
_start:


        SMSW                [MSW]


        mov                rax,[MSW]
        ror                 rax,1                        ; Check PE bit, if 1=Protected Mode, else Real Mode
        jc                 p_mode


        Print        rmsg,rmsg_len
        jmp                next


p_mode:
        Print        pmsg,pmsg_len


next:
        SGDT                [GDTR]
        SIDT                [IDTR]
        SLDT                [LDTR]
        STR                [TR]
        SMSW                [MSW]


        Print        gmsg, gmsg_len                ;GDTR (Global Descriptor Table Register)
                                                        ; LITTLE ENDIAN SO TAKE LAST WORD FIRST
        mov                 ax,[GDTR+4]                ; load value of GDTR[4,5] in ax
        call         disp16_proc                ; display GDTR contents
        mov                 ax,[GDTR+2]                ; load value of GDTR[2,3] in ax
        call         disp16_proc                ; display GDTR contents
        Print        colon,1
        mov                 ax,[GDTR+0]                ; load value of GDTR[0,1] in ax
        call         disp16_proc                ; display GDTR contents


        Print        imsg, imsg_len                ;IDTR (Interrupt Descriptor Table Register)
        mov                 ax,[IDTR+4]
        call         disp16_proc
        mov                 ax,[IDTR+2]
        call         disp16_proc
        Print        colon,1
        mov                 ax,[IDTR+0]
        call         disp16_proc


        Print        lmsg, lmsg_len                ;LDTR (Local Descriptor Table Register)
        mov                 ax,[LDTR]
        call         disp16_proc


        Print        tmsg, tmsg_len                ;TR (Task Register)
        mov                 ax,[TR]
        call         disp16_proc


        Print        mmsg, mmsg_len                ;MSW (Machine Status Word)
        mov                 ax,[MSW]
        call         disp16_proc


        Print        nline, nline_len
        Exit
;--------------------------------------------------------------------
disp16_proc:
        mov                 rbx,16                        ; divisor=16 for hex
        mov                 rcx,4                        ; number of digits
        mov                 rsi,char_ans+3                ; load last byte address of char_ans buffer in rsi


cnt:        mov                 rdx,0                        ; make rdx=0 (as in div instruction rdx:rax/rbx)
        div                 rbx


        cmp                 dl, 09h                        ; check for remainder in rdx
        jbe          add30
        add          dl, 07h
add30:
        add                 dl,30h                        ; calculate ASCII code
        mov                 [rsi],dl                        ; store it in buffer
        dec                 rsi                                ; point to one byte back


        dec                 rcx                                ; decrement count
        jnz                 cnt                                ; if not zero repeat

        Print         char_ans,4                ; display result on screen
ret
;----------------------------------------------------------------