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Old 03-19-2021, 22:53
BlackWhite BlackWhite is offline
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Can the bit0 mask in port 0x21 stop generating int 0x08?

I have submitted my bug report associated with Bochs at the following link:
https://sourceforge.net/p/bochs/patches/558/
And write a program to support my opinion. But the developers seem to
believe that when int 0x08 is masked in port 0x21, an STI instruction
can still cause int 0x08 or int 0x0F to happen. I really wonder why they
insist on PIC's generating interrupt even the associated mask is set in
port 0x21?

Here is my code to reveal the bug in Bochs:

Code:
;--------------------------------------------
;This program is written by [email protected]
;and is for the purpose of revealing the bug
;in Bochs.
;--------------------------------------------
code segment
assume cs:code, ds:code
old_01h dw 0, 0
old_08h dw 0, 0
old_09h dw 0, 0
old_0Ah dw 0, 0
old_0Fh dw 0, 0
stop    db 0
y       dw 0; y-coordiate for current output
int_08h_msg db "int_08h occurred!", 0
int_0Ah_msg db "int_0Ah occurred!", 0
int_0Fh_msg db "int_0Fh occurred!", 0
begin_msg   db "Press Esc to stop", 0
esc_msg     db "Esc is pressed.", 0
end_msg     db "Done!", 0
old_mask    db 0; original mask in port 21h
save_set_interrupts proc
   push es
   push bx
   xor bx, bx
   mov es, bx
   mov bx, 1*4
   mov ax, es:[bx]
   mov dx, es:[bx+2]
   mov old_01h[0], ax; save int_01h's interrupt vector
   mov old_01h[2], dx;
   mov word ptr es:[bx], offset int_01h
   mov es:[bx+2], cs ; set int_01h's interrupt vector
   ;
   mov bx, 8*4
   mov ax, es:[bx]
   mov dx, es:[bx+2]
   mov old_08h[0], ax; save int_08h's interrupt vector
   mov old_08h[2], dx;
   mov word ptr es:[bx], offset int_08h
   mov es:[bx+2], cs ; set int_08h's interrupt vector
   ;
   mov bx, 9*4
   mov ax, es:[bx]
   mov dx, es:[bx+2]
   mov old_09h[0], ax; save int_09h's interrupt vector
   mov old_09h[2], dx;
   mov word ptr es:[bx], offset int_09h
   mov es:[bx+2], cs ; set int_09h's interrupt vector
   ;
   mov bx, 0Ah*4
   mov ax, es:[bx]
   mov dx, es:[bx+2]
   mov old_0Ah[0], ax; save int_0Ah's interrupt vector
   mov old_0Ah[2], dx;
   mov word ptr es:[bx], offset int_0Ah
   mov es:[bx+2], cs ; set int_0Ah's interrupt vector
   ;
   mov bx, 0Fh*4
   mov ax, es:[bx]
   mov dx, es:[bx+2]
   mov old_0Fh[0], ax; save int_0Fh's interrupt vector
   mov old_0Fh[2], dx;
   mov word ptr es:[bx], offset int_0Fh
   mov es:[bx+2], cs ; set int_0Fh's interrupt vector
   pop bx
   pop es
   ret
save_set_interrupts endp

recover_interrupts proc
   push es
   push bx
   xor bx, bx
   mov es, bx
   mov bx, 1*4
   mov ax, old_01h[0]
   mov dx, old_01h[2]
   mov es:[bx], ax
   mov es:[bx+2], dx ; recover int_01h's interrupt vector
   ;
   mov bx, 8*4
   mov ax, old_08h[0]
   mov dx, old_08h[2]
   mov es:[bx], ax
   mov es:[bx+2], dx ; recover int_08h's interrupt vector
   ;
   mov bx, 9*4
   mov ax, old_09h[0]
   mov dx, old_09h[2]
   mov es:[bx], ax
   mov es:[bx+2], dx ; recover int_09h's interrupt vector
   ;
   mov bx, 0Ah*4
   mov ax, old_0Ah[0]
   mov dx, old_0Ah[2]
   mov es:[bx], ax
   mov es:[bx+2], dx ; recover int_0Ah's interrupt vector
   ;
   mov bx, 0Fh*4
   mov ax, old_0Fh[0]
   mov dx, old_0Fh[2]
   mov es:[bx], ax
   mov es:[bx+2], dx ; recover int_0Fh's interrupt vector
   ;
   pop bx
   pop es
   ret
recover_interrupts endp

output proc
   push ds
   push es
   push si
   push di
   push ax
   push dx
   ;
   push cs
   pop ds
   mov si, ax; ds:si->msg
   mov ax, 0B800h
   mov es, ax
   mov ax, 80*2
   mul cs:[y]
   mov di, ax; es:di->text on the screen
output_next_char:
   lodsb
   or al, al
   jz output_done
   stosb
   mov al, 17h; background color=blue, foreground color=white
   stosb
   jmp output_next_char
output_done:
   inc cs:[y]
   cmp cs:[y], 25
   jne screen_not_full
   mov cs:[y], 0
screen_not_full:
   pop dx
   pop ax
   pop di
   pop si
   pop es
   pop ds
   ret
output endp

int_01h:
;When this ISR is entered, TF & IF will be cleared automatically.
   push ax
   push cx
   mov al, 00h
   out 21h, al ; First we unmask all interrupts
   ;
   mov cx, 20h
next_while:
   push cx
   xor cx, cx
a_while:
   loop a_while
   pop cx
   loop next_while
               ; wait a while for the timer
   mov al, 0F9h; mask all interrupts except int_09h & int_0Ah
   out 21h, al
   sti         ; The unfixed Bochs will generate an int_08h interrupt
               ; immediately after STI is executed, while Ruppert's 
               ; patching will generate an int_0Fh interrupt instead of
               ; int_08h. But on a physical machine, neither int_08h 
               ; nor int_0Fh will be generated.
wait_key_press:
   cmp cs:[stop], 1
   nop
   jne wait_key_press
   mov ax, offset esc_msg
   call output
   mov cs:[stop], 0; reset it for next int_01h
   pop cx
   pop ax
   iret
int_08h:
   push ax
   mov ax, offset int_08h_msg
   call output
   mov al, 20h
   out 20h, al
   pop ax
   iret
int_09h:
   push ax
   in al, 60h; read key
   cmp al, 1
   jne not_esc_key
   mov cs:[stop], 1; set the signal to stop int_01h
not_esc_key:
   mov al, 20h
   out 20h, al; send EOI to PIC
   pop ax
   iret   
int_0Ah:
   push ax
   mov ax, offset int_0Ah_msg
   call output
   mov al, 20h
   out 20h, al
   pop ax
   iret
int_0Fh:
   push ax
   mov ax, offset int_0Fh_msg
   call output
   mov al, 20h
   out 20h, al
   pop ax
   iret
main:
   cli
   cld
   push cs
   pop ds
   in al, 21h
   mov old_mask, al; save mask in port 21h
   ;
   mov ax, offset begin_msg
   call output
   ;
   call save_set_interrupts
   ;
   pushf; backup FL
   ;
   pushf
   pop ax
   or ax, 0100h; TF=1
   push ax
   popf
   nop         ; There will be an int_01h after this nop.
;int_01h
   popf        ; recover FL, TF=0
;int_01h
   nop
;no more int_01h
   cli
   call recover_interrupts
   sti
   mov ax, offset end_msg
   call output
   mov al, old_mask
   out 21h, al; recover mask in port 21h
   mov ah, 4Ch
   int 21h
code ends
end main
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