Page 186 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes
016 inc and jmp bx bx, ; increment to next shape; taking
017 3taskone modulus by 4; infinite task
018
019

020 ; second task to be multitaskedtasktwo: ; read the next shape; write at top
021 mov al, [chars+bx]mov [es:158], alinc bx right of screen; increment to next
022 and bx, 3jmp tasktwo shape; taking modulus by 4;
023 infinite task
024
025
026
027 ; timer interrupt service routinetimer:
028 push axpush bx
029
030
031 mov mov mul mov bl, [cs:current]ax, ; read index of current task;
032 10bl bx, ax space used by one task;
033 multiply to get start of task;
034 load start of task in bx
035
036 popmov popmov ax ; read original value of bx[cs:taskstates+bx+2], ax ;
037 popmov popmov space for current taskax ; read original value of
038 popmov ax[cs:taskstates+bx+0], ax ; space for current taskax ;
039 read original value of ip[cs:taskstates+bx+4], ax ;
040 space for current taskax ; read original value of
041 cs[cs:taskstates+bx+6], ax ; space for current taskax ;
042 read original value of flags[cs:taskstates+bx+8], ax ;
043 space for current task
044
045
046

047 inc byte [cs:current] ; update current task indexcmp byte [cs:current], 3 ; is task
048 index out of rangejne skipreset ; no, proceedmov byte [cs:current], 0 ; yes, reset
049 to task 0
050
051

052 skipreset: mov bl, [cs:current]ax, ; read index of current task;
053 mov 10bl bx, ax space used by one task;
054 mul multiply to get start of task;
055 mov load start of task in bx
056

057 mov al, 0x200x20, al ; send EOI to PIC
058 out
059
060 push word [cs:taskstates+bx+8] ; flags of new taskpush word
061 [cs:taskstates+bx+6] ; cs of new taskpush word [cs:taskstates+bx+4] ; ip of new
180 062 LOVELY PROFESSIONAL UNIVERSITY
taskmov ax, [cs:taskstates+bx+0] ; ax of new task mov bx, [cs:taskstates+bx+2] ;
063 bx of new taskiret ; return to new task
064
065
066

067 start: mov mov word [taskstates+10+4], taskone ; initialize
068 mov mov ip[taskstates+10+6], cs ; initialize csword
069 mov mov [taskstates+10+8], 0×0200 ; initialize flagsword
070 mov [taskstates+20+4], tasktwo ; initialize
071 ip[taskstates+20+6], cs ; initialize csword
072 [taskstates+20+8], 0×0200 ; initialize flagsword
073 [current], 0 ; set current task index
074
075 xor mov cli ax, ax es, ax ; point es to IVT base
076
077
078 mov mov mov mov word [es:8*4], ; hook timer interrupt ;
079 xor sti timer[es:8*4+2], csax, point es to video base;