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flat assembler > DOS > bootable usb and task switch

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andr.dimitris



Joined: 10 Feb 2012
Posts: 2
bootable usb and task switch
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:
stacktask1rb STACKSIZE   
stack1end:                  
stacktask2rb 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 0x20al
                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
Post 10 Feb 2012, 14:19
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edfed



Joined: 20 Feb 2006
Posts: 4095
Location: brighter future, please.
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.
Post 10 Feb 2012, 17:15
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andr.dimitris



Joined: 10 Feb 2012
Posts: 2
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.
Post 10 Feb 2012, 17:25
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