Blog: CS401 Assignment No. 4 solution

; elementary multitasking of two threads
[org 0x0100]


jmp start


       ; ax,bx,ip,cs,flags storage area
taskstates: dw 0, 0, 0, 0, 0 ; task0 regs
dw 0, 0, 0, 0, 0 ; task1 regs
dw 0, 0, 0, 0, 0 ; task2 regs
dw 0, 0, 0, 0, 0 ; task3 regs


current: db 0 ; index of current task


chars: db '\|/-' ; shapes to form a bar


; first task to be multitasked

taskone: mov al, [chars+bx] ; read the next shape
mov [es:0], al ; write at top left of screen
inc bx ; increment to next shape
and bx, 3 ; taking modulus by 4
jmp taskone ; infinite task


; second task to be multitasked

tasktwo: mov al, [chars+bx] ; read the next shape
mov [es:158], al ; write at top right of screen
inc bx ; increment to next shape
and bx, 3 ; taking modulus by 4
jmp tasktwo ; infinite task


; third task to be multitasked

taskthree: mov al, [chars+bx] ; read the next shape
mov [es:80], al ; write at middle of screen
inc bx ; increment to next shape
and bx, 3 ; taking modulus by 4
jmp taskthr ; infinite task




; timer interrupt service routine


timer: push ax
push bx
mov bl, [cs:current] ; read index of current task
mov ax, 10 ; space used by one task
mul bl ; multiply to get start of task
mov bx, ax ; load start of task in bx
pop ax ; read original value of bx
mov [cs:taskstates+bx+2], ax ; space for current task
pop ax ; read original value of ax
mov [cs:taskstates+bx+0], ax ; space for current task
pop ax ; read original value of ip
mov [cs:taskstates+bx+4], ax ; space for current task
pop ax ; read original value of cs
mov [cs:taskstates+bx+6], ax ; space for current task
pop ax ; read original value of flags
mov [cs:taskstates+bx+8], ax ; space for current task
inc byte [cs:current] ; update current task index
cmp byte [cs:current], 4 ; is task index out of range
jne skipreset ; no, proceed
mov byte [cs:current], 0 ; yes, reset to task 0


skipreset: mov bl, [cs:current] ; read index of current task
mov ax, 10 ; space used by one task
mul bl ; multiply to get start of task
mov bx, ax ; load start of task in bx
mov al, 0x20
out 0x20, al ; send EOI to PIC
push word [cs:taskstates+bx+8] ; flags of new task
push word [cs:taskstates+bx+6] ; cs of new task
push word [cs:taskstates+bx+4] ; ip of new task
mov ax, [cs:taskstates+bx+0] ; ax of new task
mov bx, [cs:taskstates+bx+2] ; bx of new task
iret ; return to new task


start: mov word [taskstates+10+4], taskone ; initialize ip
mov [taskstates+10+6], cs ; initialize cs
mov word [taskstates+10+8], 0x0200 ; initialize flags


mov word [taskstates+20+4], tasktwo ; initialize ip
mov [taskstates+20+6], cs ; initialize cs
mov word [taskstates+20+8], 0x0200 ; initialize flags


mov word [taskstates+30+4], taskthree ; initialize ip
mov [taskstates+30+6], cs ; initialize cs
mov word [taskstates+30+8], 0x0200 ; initialize flags


mov word [current], 0 ; set current task index
xor ax, ax
mov es, ax ; point es to IVT base
cli
mov word [es:8*4], timer
mov [es:8*4+2], cs ; hook timer interrupt
mov ax, 0xb800
mov es, ax ; point es to video base
xor bx, bx ; initialize bx for tasks
sti


jmp $ ; infinite loop