Solution to

1
Solution to 10, Page 115

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
The Problem

• Prompt for single Hex digit
• '0' through '9', 'A' through 'F', 'a' through 'f'
• Illegal entries must be detected
• Display entry in decimal
• 0 through 15
• Ask to repeat the program
• 'y' or 'Y' will repeat, anything else exits

3
Approach

• The major task is data translation
• Given an ASCII code for a Hex digit, determine
  the numeric equivalent (unsigned byte)
• Given an unsigned byte (0-15), display one or two
  character decimal representation
• The input/process/output step must be embedded in
  a do until loop controlled by the yes/no response
  of the user

4
Prompted Input

• Display prompt on screen
• DOS function 9 is convenient to do this
• The prompt should be terminated with a
• Wait for single character input
• DOS function 1 will do this, returning with the
  ASCII code of the input character in AL and
  leaves the cursor to the right of the entered
  character
• Output CR/LF

5
Code Detail - Input

• begin_process
• prompt and accept single char input
• lea dx,input_message
• mov ah,9 display input prompt
• int 21h
• mov ah,1 DOS input function
• int 21h get single char response
• mov bl,al for safe keeping
• lea dx,output_message
• mov ah,9 move to new line and get
• int 21h ready to display result

6
Hex to Unsigned Conversion

• Given a character, we divide the task into two
  parts
• If entry is '0'-'9'
• subtract '0' to get unsigned byte
• 'A'-'F' or 'a'-'f'
• convert to uppercase by subtracting ('a'-'A')
• subtract ('A'-10) to get unsigned byte
• Invalid data is detected after conversion

7
Code Detail - Hex to Unsigned

• convert ASCII hex digit (bl) to unsigned
• cmp bl,'9' check if decimal digit
• jle decimal_size
• cmp bl,'F' uppercase or lowercase?
• jle uppercase
• sub bl,'a'-'A' lower to uppercase
• uppercase handle 'A'-'F' case
• sub bl,'A'-10 'A'-'F' to unsigned
• jmp short conversion_complete
• decimal_size handle '0'-'9' case
• sub bl,'0' convert to unsigned
• conversion_complete

8
Detecting Illegal Input

• After applying the conversion algorithm, we
  should have a byte in the range 0-15
• If illegal data was entered, the result will fall
  outside this range
• If illegal, display an error message and jump
  back to the top of the program to allow the user
  another chance

9
Code Detail - Illegal Input

• verify byte in BL is legal (0-15)
• cmp bl,0 if lt0, illegal
• jl illegal
• cmp bl,15 here if gt0
• jle legal if also lt15, OK
• illegal
• mov ah,9 display error message
• lea dx,illegal_message
• int 21h
• jmp begin_process start over
• legal

10
Unsigned to Decimal

• Given an unsigned byte (0-15) we break the task
  into three steps
• If 10 or more, print a '1' and subtract 10
• Convert the byte (0-9) to ASCII by adding '0'
• Display the ASCII character

11
Code Detail - Unsigned to Decimal

• output byte in BL (0-15) as decimal
• mov ah,2 DOS output function
• cmp bl,10
• jl ones_place jmp if single digit
• mov dl,'1' output leading '1'
• int 21h
• sub bl,10 isolate ones place
• ones_place
• add bl,'0' convert to ASCII
• mov dl,bl prepare to output
• int 21h

12
Outer Control Loop

• A label marks the top of the loop
• At the end of the processing task
• Prompt user to repeat the task
• Accept single character input
• If entry is 'y' or 'Y', jump to top of loop
• Otherwise, exit

13
Code Detail - Outer Loop

• begin_process
• main processing task goes here
• lea dx,continue_message
• mov ah,9 display continue prompt
• int 21h
• mov ah,1
• int 21h get response
• cmp al,'y' y means do again
• je begin_process
• cmp al,'Y' Y means do again
• je begin_process

14
Data Elements

• The only data we need to declare are the strings
  for output
• Place a at the end of each string so we can use
  DOS output function 9
• Embed CR/LF's at strategic locations for line
  control

15
Code Detail - Data Segment

• .data
• input_message db 13,10
• db 'Enter a legal hex digit '
• output_message db 13,10
• db 'in decimal this is '
• illegal_message db 'Illegal entry!'
• continue_message db 13,10
• db 'Try again? (Y/N) '

16
Putting It All Together

• Add standard entrance and exit code
• DS must be set to the start of the actual data
  segment so we can access the strings
• When program is finished, we must exit gracefully
  to DOS
• Remember the model and stack directives
• See Hexable.asm for source file

17
Code Detail - Standard Entrance/Exit

• .code
• hexable proc
• mov ax,_at_data ax gets segment number
• mov ds,ax transfer to segment reg
• program code goes here
• mov ax,4c00h DOS return, no error
• int 21h
• hexable endp
• end hexable