Question

I have an assignment that is described like so:

- Reads printable characters (20h-7Fh) from the keyboard without echoing
- Uppercase letters are printed to display
- Lowercase letters are converted to uppercase and then printed to display
- Blank spaces and periods are printed to display; everything else is trashed
- Program ends when period is printed

My program so far is this:

    .model      small
    .8086


    .data


    .code

start:
        mov     ax,@data
        mov     ds,ax

read:
        mov     ah,8
        int     21h
        cmp     al,61
        jl      write
        cmp     al,7fh
        jg      read
        sub     al,20h
        jmp     write

write:      cmp     al,20h
        jl      read
        mov     dl,al
        mov     ah,2
        int     21h
        cmp     dl,2eh
        jne     read

exit:
        mov     ax,4c00h
        int     21h
        end     start

My program succesfully converts lowercase letters and prints the corresponding uppercase letter, but I am having trouble trashing everything else. What is the best way to only allow blank spaces, periods, and letters through to the display?

Looking at the ASCII chart,

21h - 2Dh can be trashed
2Fh - 40h can be trashed
5bh - 60h can be trashed
7bh - 7fh can be trashed

Can anyone help me come up with the best logic for comparing the input to these values and then trashing those that fall between the range above? We are graded on efficiency, with 0-20 instructions written awarded full credit. I am already at 20 instructions here and I haven’t included compares to find the trash values.

EDIT
Here is what I have narrowed my code down to:

    .model      small
    .8086


    .data


    .code


read:
        mov     ah,8
        int     21h
        cmp     al,' '
        je      write
        cmp     al,'.'
        je      write
        cmp     al,'a'
        jl      read
        cmp     al,'Z'
        jg      convert

convert:    
        cmp     al,'a'
        jl      read
        sub     al,20h

write:      
        mov     dl,al
        mov     ah,2
        int     21h
        cmp     dl,'.'
        jne     read

exit:
        mov     ax, 4c00h
        int     21h
        end     read

Currently at 21 instructions! Is there any redundancy in my code that can be removed to get it down to 20?

Answer 1

You might modify your logic to do something like:

• if space or period, jump to write
• fold lowercase to uppercase
• if not uppercase letter, jump back to read

Note that the “fold lowercase to uppercase” doesn’t need to check to see whether the input was a letter first. Just and xx where xx is an appropriate value should do it (I’ll let you work out the value). This step will also modify characters other than letters, but since the next step is to check for a letter it doesn’t matter.