OpenU 80286 Assembly exercise, July 2008 Raw

1501.asm

;; MAMAN 15
;; Question 1

P286N
IDEAL
DOSSEG
MODEL SMALL
STACK 100h

MAXIMUM = 20                   ; Maximum characters to read off the keyboard

DATASEG

CODESEG
 ; Shared variables
 caller         db      'p'     ; 'm' means main program, 'p' means procedure
 charCount      db      0       ; How many characters have been read
 old_isr1ch     dd      ?       ; Reference to the old 1Ch interrupt

 ; INTERRUPT 1CH
 ; Our ISR1Ch
 PROC myisr1ch FAR
        pusha
        push es                 ; Stores the es register seperately

        mov dl, [caller]        ; 'caller' is initialized with 'p'
        cmp dl, 'm'             ; so the procedure will run only after the
        jne @@ret               ; main program has run. So we check it here

        xor ax, ax

        push 40h
        pop es                  ; ES points at keyboard queue

        @@ReadQueue:
        mov bx, [es:1ah]        ; BX = head

        @@polling:
                cmp bx, [es:1ch]; Is the queue empty?
                je @@polling

                mov al, [es:bx] ; AL contains character

                add bx, 2       ; Moving on to the next character in queue
                cmp bx, [es:82h]; Are we at the end of the queue?
                jb @@continue_print

        @@reset_head:          ; Progresses the pointer to beginning of queue
                mov bx, [es:80h]

        @@continue_print:
                mov [es:1ah], bx; Updating the top of queue
                push 0b800h
                pop ds          ; Printing the character
                mov [ds:(25*80)*2], al
                mov [ds:((25*80)*2)+1], 4;The red color

        ; This happens after a character was read
        @@UpdateVariables:
        mov dl, [CharCount]
        add dl, 1
        mov [CharCount], dl     ; Increase the character count by one

        mov dl, 'p'
        mov [caller], dl        ; Set procedure as last caller

        @@ret:
        pop es
        popa

        jmp [dword old_isr1ch]  ; Call old interrupt
 ENDP myisr1ch

START:
        mov ax, @data
        mov ds, ax

        ; As the question prohibits me from accessing the screen using
        ; BIOS/DOS interrupts, I cannot alter the screen mode. So I
        ; assume the screen is in text mode.

        ; Store the old interrupt reference
        mov al, 1ch
        mov ah, 35h
        int 21h

        mov [word cs:old_isr1ch], bx    ; Offset is in BX
        mov [word cs:old_isr1ch+2], es  ; Segment is in ES

        ; Modify the interrupt reference
        mov ah, 25h
        mov al, 1ch
        mov dx, offset myisr1ch
        push seg myisr1ch
        pop ds
        int 21h

        xor ax, ax

        push 40h
        pop es                  ; ES points at keyboard queue

        ReadFromQueue:
        mov bx, [es:1ah]        ; BX = head

        @@polling:
                cmp bx, [es:1ch]; Is the queue empty?
                je @@polling

                mov dl, [caller]
                cmp dl, 'p'     ; Check if the caller is the procedure
                jne @@ContinueWithoutPrinting

                mov al, [es:bx] ; AL contains character

                ; Moving on to the next character in queue
                add bx, 2
                cmp bx, [es:82h]; Are we at the end of the queue?
                jb @@continue_print

        @@reset_head:           ; If so, we'll progress the pointer to the
                mov bx, [es:80h]; beginning of the queue

        @@continue_print:
                mov [es:1ah], bx; Updating the top of queue
                push 0b800h
                pop ds          ; Print out the character
                mov [ds:0], al
                mov [ds:1], 14  ; The yellow color

        ; This happens after a character was read
        @@ContinueWithoutPrinting:
        mov dl, [CharCount]
        add dl, 1
        mov [CharCount], dl     ; Increase the character count by one

        mov dl, 'm'
        mov [caller], dl        ; Set main program as last caller

        ; Waits for the procedure to read next character
        WaitForProcedure:
        mov dl, [caller]
        cmp dl, 'p'
        jne WaitForProcedure

        ; Procedure has finished running, check if we hit our MAXIMUM limit
        mov dl, [CharCount]
        cmp dl, MAXIMUM
        je EXIT                 ; Our limit has been hit, finish the program
        jmp ReadFromQueue       ; There are some more characters to read

        ; Finish the program
EXIT:
        mov ax, 4c00h
        int 21h
End START