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         ;