CLI and STI information or explanation

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vbVeryBeginner

Joined: 15 Aug 2004
Posts: 884
Location: \\world\asia\malaysia

vbVeryBeginner

according to a book i read:
CLI: Clear Interrupt Flag
Operation: Clears the IF, to disable maskable external interrupts. (see also STI)

STI: Set Interrupt Flag
Operation: Sets the IF to 1 to enable maskable external interrupts after execution of the next instruction. (see CLI for clear IF)

and i check with the index for interrupt flag and found

IF (Interrupt). Indicates that all external interrupts, such as keyboard entry, are to be processed or ignored.

IF (Interrrupt Flag). Disables interrupts when 0, and enables interrupts when 1. This flag is rarely used in conventional programming.

and i saw from the privalov tetros source

Code:

        cli
        xor     ax,ax
        mov     ss,ax
        mov     ds,ax
        mov     es,ax
        mov     sp,0FFFEh
        sti
        push    ax
        push    start
        retf
start:

the thing i am confused is, why we need to CLI when we want to move value into segment registers? and

does this mean, external interrupts (such as keyboard) will make use of our segment registers? upon booting up the OS?

and the solor os also resemble something like this

Code:

;------------------------------------------------------------
; back to code...
; BIOS loads this first  512bytes sector at 0000:7C00h
;-------------------------------------------------------------
boot02:
     cli                     ; please no interupts
;      mov     ax,07C0h
    mov     ax,cs
       mov     ds,ax
       mov     es,ax           ; es same as ds

 mov     [boot_drv],dl   ; save drive #
;------------------------------------------------------
; here we make our own stack
; just under the A000h video memory
;------------------------------------------------------
 mov     ax,09000h       ; segment for stack
 mov     ss,ax
       mov     sp,0F800h       ; offset for stack

      sti                     ; enable ints again

and menuet os doesn't have such instruction in its first 512 bytes

sincerely
me

23 Oct 2004, 23:14

Chewy509

Joined: 19 Jun 2003
Posts: 297
Location: Bris-vegas, Australia

Chewy509

The cli/sti pair are being used because they are modifying the Stack Segment (SS) and Stack Pointer (SP) registers. If an interrupt occurs mid-stack segment/pointer change, it's possible that the system will crash. Any Interrupt Service Routines (ISRs), will use your stack (either in full or just briefly) while servicing the IRQ. So it's really important that a valid stack is accessible while external interrupts are enabled.

The safe bet to just to disable external int's, until you have your stack setup.

24 Oct 2004, 07:51

vbVeryBeginner

Joined: 15 Aug 2004
Posts: 884
Location: \\world\asia\malaysia

vbVeryBeginner

thank you,
i guess they should put your statement in book :p

24 Oct 2004, 19:46

ASHLEY4

Joined: 28 Apr 2004
Posts: 376
Location: UK

ASHLEY4

Your book sounds like :"IBM PC Assembly Language and Programming by Peter Abel" ?.

\\\\||////
(@@)
ASHLEY4.

25 Oct 2004, 02:10

vbVeryBeginner

Joined: 15 Aug 2004
Posts: 884
Location: \\world\asia\malaysia

vbVeryBeginner

ASHLEY4 wrote:

Your book sounds like :"IBM PC Assembly Language and Programming by Peter Abel" ?.

yup, how you know? you own that book also? i got a Fourth Edition here, it is by Peter Abel
Professor Emeritus
British Columbia
Institute of Technology

don't tell me you are Peter Abel??
if it is,
nice to met you Smile

i like people who got lot of knowledge Smile

25 Oct 2004, 03:38

ASHLEY4

Joined: 28 Apr 2004
Posts: 376
Location: UK

ASHLEY4

No just have the book, have you read this book ?.
http://www.cs.smith.edu/~thiebaut/ArtOfAssembly/artofasm.html
you can ask Randall Hyde ? here: http://board.win32asmcommunity.net/forum.php?f=30

\\\\||////
(@@)
ASHLEY4.

25 Oct 2004, 14:24

Octavio

Joined: 21 Jun 2003
Posts: 366
Location: Spain

Octavio

this instruction: mov ss,ax
disables interrupts until the end of next instruction
to make possible to load ss and sp in two consecutive instructions without disabling interrupts.

25 Oct 2004, 19:58

Chewy509

Joined: 19 Jun 2003
Posts: 297
Location: Bris-vegas, Australia

Chewy509

Octavio wrote:

this instruction: mov ss,ax
disables interrupts until the end of next instruction
to make possible to load ss and sp in two consecutive instructions without disabling interrupts.

I thought that was implementation specific and couldn't be relied upon in all cases? (Please correct me if am wrong).

25 Oct 2004, 23:18

Gomer73

Joined: 29 Nov 2003
Posts: 151

Gomer73

It should be universally compatible after whichever processor implemented it(386, 486, I forget which one.)

But it is still a pain, because you would have to load another register prior to the move. The following wouldn't work:

mov ss,ax
mov ax,0xf000
mov sp,ax

you would need to do something like:

mov bx,0xf000
mov ss,ax
mov sp,bx

26 Oct 2004, 17:34

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8026
Location: Kraków, Poland

Tomasz Grysztar

You can also use "LSS SP,[pointer_variable]" instruction to load both SS and SP with one instruction.

26 Oct 2004, 20:38

Octavio

Joined: 21 Jun 2003
Posts: 366
Location: Spain

Octavio

Gomer73 wrote:

The following wouldn't work:

mov ss,ax
mov ax,0xf000
mov sp,ax

but the following will work (+386)

mov ss,ax
mov sp,0f000h

26 Oct 2004, 22:07

Gomer73

Joined: 29 Nov 2003
Posts: 151

Gomer73

Makes since, I forget what instructions can be used where.

Thanks,

...Gomer73

27 Oct 2004, 15:51

neonz

Joined: 02 Aug 2003
Posts: 62
Location: Latvia

neonz

Actually, CLI/STI on modifying stack is only necessary for 8086/8088 CPUs. All newer Intel CPUs AFAIK do that CLI/STI thing automatically.

06 Nov 2004, 18:02

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