I want to do task switching using two tasks while i have booted from usb drive. The first switch takes place but the second one does not happen. Any ideas why? the code is below. The routine which does the task switching is at label sched. For the boot i used mike gonta's code.

Code:

org 100h jmp start STACKSIZE       dw 256 timer:          dd 0 active1:        db 1 active2:        db 1 vidmem_start       equ 0x0b800 default_color    equ 7           ;default_color scrw          equ 80*25       ;25=lines 80=chars per line eom              equ 0 stat1       dw 0 stat2       dw 0 stasys      dw 0 temp        dw ?       oldIntOff:      dw 0                ; old timer interrupt offset oldIntSeg:      dw 0                  stacksys:  rb  STACKSIZE stacksysend: stacktask1: rb STACKSIZE    stack1end:                   stacktask2: rb STACKSIZE  stack2end: start:          mov sp,stack2end                           pushf                                 push cs                                       push task2                                  pusha                        mov ax,sp            mov [stat2],ax               mov sp,stack1end                 pushf            push cs              push task1           pusha                mov [stat1],sp                                                                         mov sp,stacksysend           push word [stat2]                  mov bp,sp              mov [stasys],sp              mov sp,[stat1]                                                                                                      mov ax,0             mov gs,ax            mov ax,[gs:0x1c*4]               mov [oldIntOff],ax           mov ax,[gs:0x1c*4+2]             mov [oldIntSeg],ax         ; Critical Section Start: Hook the new ISR for 0x1c                cli          mov ax,cs            mov [gs:0x1c*4+2],ax             mov ax,timer_isr             mov [gs:0x1c*4],ax       ; Critical Section End: ISR is hooked                            sti                                                                     main:                    task1:                       mov bx,vidmem_start                        mov ax,0                 mov ds,bx                 mov di,0                 mov ah,6                 mov al,2 a1:                              mov [ds:di],ax                inc di               mov cx,0FFh          loop $               cmp di,4000                 jnz a1                mov [active1],byte 0 task1end:       jmp task1end   task2:          mov bx,vidmem_start                       mov ax,0                 mov ds,bx                 mov di,3998                 mov ah,6                 mov al,3 a2:                              mov [ds:di],ax                 dec di               mov cx,0fFh          loop $               CMP di,4000                 jnz a2                        mov [active2],byte 0 task2end:       jmp task2end                                                               timer_isr:      cmp [active1],byte 0                   jnz sch            cmp [active2],byte 0                 jnz sch              jmp end1                              sch:            push ax              mov ax,cs            mov ds,ax                 inc dword [timer]               mov al,0x20                        out 0x20, al                 pop ax                                                                                                                                                                                                          ; the task switch takes place here                                          pusha                                                 mov [temp],sp                                                                      mov sp,[stasys]                                           push word [temp]                                          mov [stasys],sp                                           mov sp,[bp]                       sub bp,2                                                                                                    popa                                 ;pushf                               ;call FAR [oldIntOff]                                iret                                                                                                                                                                                                                                                                            end1  :         cli                          mov ax,[oldIntOff]                                           mov [gs:0x1c*4],ax                               mov ax,[oldIntSeg]                           mov [gs:0x1c*4+2],ax                             sti                                                           int 0x21         ;

Edit by revolution - fixed code tags

task switch is not so simple. everything should be saved, even the stack of the task.
and each task have it's own stack, otherwise, it will not work.

yes, i do all of these. it actually works for the first switch but thes it does not do a second one. I use int1c for the switching; each time the int occurs, i switch. I believe it has something to do with the scheduler but I am not so sure, as for the first switch it works fine.