CS401 assignment no 4 Solution Fall 2010

Assignment


Modify multitasking and dynamic thread registration example (11.2) such that
• It displays a rotating bar on bottom left corner of the screen (you can take help from Example 11.1).
• Also, if we press “S (both small and caps)”, it will generate new process which will display rotating bars on screen in next row from the previous one.
• And if “A (both small and caps)” is pressed from keyboard it will restore every thing to normal as it was before the multitasking kernel was there.
• You can generate maximum up to 10 processes. After 10 processes no new process will be created if “S (both small and caps)” will be pressed from keyboard.

Note: comments and indentations with each line in your code should be properly used it has 2 marks.

[org 0x0100]
jmp start
; Original Registers:
;ax, bx, cx, dx, si, di, bp, sp, ip, cs, ds, ss, es, flags
; 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26
; PCB layout:
;ax, bx, cx, dx, si, di, bp, sp, ip, cs, ds, ss, es, flags, next, dummy
; 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26 , 28 , 30
originalstate: times 14 dw 0
oldtimer: dd 0
pcb: times 20*16 dw 0 ; space for 20 PCBs
stack: times 20*256 dw 0 ; space for 20 512 byte stacks
nextpcb: dw 1 ; index of next free pcb
current: dw 0 ; index of current pcb
column_no: dw 0 ; column for next thread
chars: db ‘/-\|’ ; shapes to form a bar
mytask:
push bp ;save registers value
mov bp, sp
push ds
push es
push di
push ax
push bx
mov ax, 0xb800
mov es, ax ; point es to video base
push cs
pop ds ; point ds to current segment
mov di, [bp+4] ; load ax with column number
shl di, 1 ; turn into byte count
and bx, 0 ; initialize bx
mov ah, 0×07 ; normal attribute
rotation:
mov al, [chars+bx] ; read current shape
mov [es], ax ; print at specified place
inc bx ; update for next shape
and bx, 3 ; take modulus with 4
jmp rotation ; repeat infinitely
pop di ; restore register values
pop bx
pop ax
pop es
pop ds
pop bp
ret
initpcb:
push bp ; save registers
mov bp, sp
push ax
push bx
push cx
push si
mov bx, [nextpcb] ; read next available pcb index
cmp bx, 20 ; are all PCBs used
je exit ; yes, exit
mov cl, 5
shl bx, cl ; multiply by 32 for pcb start
mov ax, [bp+8] ; read segment parameter
mov [pcb+bx+18], ax ; save in pcb space for cs
mov ax, [bp+6] ; read offset parameter
mov [pcb+bx+16], ax ; save in pcb space for ip
mov [pcb+bx+22], ds ; set stack to our segment
mov si, [nextpcb] ; read this pcb index
mov cl, 9
shl si, cl ; multiply by 512
add si, 256*2+stack ; end of stack for this thread
mov ax, [bp+4] ; read parameter for subroutine
sub si, 2 ; decrement thread stack pointer
mov [si], ax ; pushing param on thread stack
sub si, 2 ; space for return address
mov [pcb+bx+14], si ; save si in pcb space for sp
mov word [pcb+bx+26], 0×0200 ; initialize thread flags
mov ax, [pcb+28] ; read next of 0th thread in ax
mov [pcb+bx+28], ax ; set as next of new thread
mov ax, [nextpcb] ; read new thread index
mov [pcb+28], ax ; set as next of 0th thread
inc word [nextpcb] ; this pcb is now used
exit:
pop si ; restore all registers
pop cx
pop bx
pop ax
pop bp
ret 6 ;discard 3 parameters from stack
timer:
push ds
push bx
push cs
pop ds ; initialize ds to data segment
mov bx, [current] ; read index of current in bx
shl bx, 5 ; multiply by 32 for pcb start
mov [pcb+bx+0], ax ; save ax in current pcb
mov [pcb+bx+4], cx ; save cx in current pcb
mov [pcb+bx+6], dx ; save dx in current pcb
mov [pcb+bx+8], si ; save si in current pcb
mov [pcb+bx+10], di ; save di in current pcb
mov [pcb+bx+12], bp ; save bp in current pcb
mov [pcb+bx+24], es ; save es in current pcb
pop ax ; read original bx from stack
mov [pcb+bx+2], ax ; save bx in current pcb
pop ax ; read original ds from stack
mov [pcb+bx+20], ax ; save ds in current pcb
pop ax ; read original ip from stack
mov [pcb+bx+16], ax ; save ip in current pcb
pop ax ; read original cs from stack
mov [pcb+bx+18], ax ; save cs in current pcb
pop ax ; read original flags from stack
mov [pcb+bx+26], ax ; save cs in current pcb
mov [pcb+bx+22], ss ; save ss in current pcb
mov [pcb+bx+14], sp ; save sp in current pcb
mov bx, [pcb+bx+28] ; read next pcb of this pcb
mov [current], bx ; update current to new pcb
mov cl, 5
shl bx, cl ; multiply by 32 for pcb start
mov cx, [pcb+bx+4] ; read cx of new process
mov dx, [pcb+bx+6] ; read dx of new process
mov si, [pcb+bx+8] ; read si of new process
mov di, [pcb+bx+10] ; read di of new process
mov bp, [pcb+bx+12] ; read bp of new process
mov es, [pcb+bx+24] ; read es of new process
mov ss, [pcb+bx+22] ; read ss of new process
mov sp, [pcb+bx+14] ; read sp of new process
push word [pcb+bx+26] ; push flags of new process
push word [pcb+bx+18] ; push cs of new process
push word [pcb+bx+16] ; push ip of new process
push word [pcb+bx+20] ; push ds of new process
mov al, 0×20
out 0×20, al ; send EOI to PIC
mov ax, [pcb+bx+0] ; read ax of new process
mov bx, [pcb+bx+2] ; read bx of new process
pop ds ; read ds of new process
iret ; return to new process
start:
mov [originalstate], ax ; save original state
mov [originalstate +2], bx
mov [originalstate +4], cx
mov [originalstate +6], dx
mov [originalstate +8], si
mov [originalstate +10], di
mov [originalstate +12], bp
mov [originalstate +14], sp
mov [originalstate +20], ds
mov [originalstate +22], ss
mov [originalstate +24], es
pushf ; push flags on stack
pop word [originalstate +26] ; save flags register in memory
xor ax, ax ; initialize ax with zero
mov es, ax ; point es to IVT base
mov bx, [es*4] ; load offset of timer ISR
mov word [oldtimer], bx ; save original ISR offset
mov bx, [es*4+2] ; load segment of timer ISR
mov [oldtimer +2], bx ; save original ISR segment
cli ; disable all interrupts
mov word [es*4], timer ; hooking timer interrupt
mov [es*4+2], cs
sti ; enable interrupts
nextkey:
xor ah, ah ; service 0 – get keystroke
int 0×16 ; bios keyboard services
cmp al, ‘s’ ; compeare if s is pressed
je newtask ; if s is pressed go to newtask
cmp al, ‘S’ ; compeare if S is pressed
je newtask ; if S is pressed go to newtask
cmp al, ‘q’ ; compeare if q is pressed
je quit ; if q is pressed go to quit
cmp al, ‘Q’ ; compeare if Q is pressed
je quit ; if Q is pressed go to quit
jmp nextkey ; otherwise wait for next key
newtask:
push cs ; use current code segment
mov ax, mytask
push ax ; use mytask as offset
push word [column_no] ; Pushing [column_no] as thread parameter
call initpcb ; register the thread
inc word [column_no] ; increment column_no by 1
jmp nextkey ; wait for next keypress
quit:
xor ax, ax
mov es, ax ; point es to IVT base
cli ; disable interrupts
mov ax, word [oldtimer] ; load timer offset in ax
mov [es*4], ax ; Point IVT to original timer ISR
mov ax, word [oldtimer+2] ; load timer segment in ax
mov [es*4+2], ax ; Point IVT to original timer ISR
sti ; enable interrupts
mov ax, [originalstate] ; restore original state
mov bx, [originalstate +2]
mov cx, [originalstate +4]
mov dx, [originalstate +6]
mov si, [originalstate +8]
mov di, [originalstate +10]
mov bp, [originalstate +12]
mov sp, [originalstate +14]
mov ds, [originalstate +20]
mov ss, [originalstate +22]
mov es, [originalstate +24]
push word [originalstate +26]
popf ; restore flag register
mov ah, 0x4c ; DOS service to terminate program
int 0×21