Bcd And Ascii Code 2y374c

BCD Numbers and ASCII Code BCD Numbers Binary Coded Decimal (BCD) as the name implies is a way of representing Decimal numbers in a 4 bit binary code. BCD numbers are useful when sending data to display devices for example. The numbers 0 through 9 are the only valid BCD values. Notice in the table that the binary and BCD values are the same for the numbers 0 to 9. When we exceed the value of 9 in BCD each digit in the BCD number is now represented by a 4 bit binary value. Number 0 1 2 3 4 5 6 7 8 9 10 11

Binary 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 – invalid BCD number 1011 – invalid BCD number

BCD 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 0001 0000 0001 0001

ASCII Code Computers must have some way to represent the common symbols (letters, numbers, and other characters) and store them in memory. ASCII code was developed to do this task. ASCII code tables are easy to find on the Internet. A 7 bit ASCII code is the only standardized version of this code. There are 8 bit ASCII codes but none has been adopted as an official standard. Unicode is a 16 bit extension of ASCII code. Some simple rules exist for understanding ASCII code Letters in ASCII Code Letters are represented sequentially – for example Letter “A” “B” “C” etc

Upper Case values in hex 41 42 43

Lower Case values in hex 61 62 63

The difference between the hex values for a lower case letter and the same upper case letter is always 20 hex. Lower case to upper case conversion can be done in Assembler ldaa anda

#’a’ #$DF

ldaa suba

#’a’ #$20

;eora

#$20 also works

OR

Exercise: Write the code to convert Upper case to Lower case. Numbers in ASCII Code Numbers are represented in ASCII by adding 30 in hex to the value of the number. The ASCII code for 9 is 39. Computer keyboards generate the ASCII code for a number when pressed. Key 0 1 2 3 4 5 6 7 8 9

ASCII value 30 31 32 33 34 35 36 37 38 39

ASCII value in Binary 0011 0000 0011 0001 0011 0010 0011 0011 0011 0100 0011 0101 0011 0110 0011 0111 0011 1000 0011 1001

BCD value 0000 0000 0000 0001 0000 0010 0000 0011 0000 0100 0000 0101 0000 0110 0000 0111 0000 1000 0000 1001

An ASCII value in binary can be converted to a BCD value in Assembler in two ways Assume A contains ASCII value in binary. The following methods convert it to BCD Method 1 ldaa anda

#$35 #$0F

ldaa suba

#$35 #$30

Method 2

Packed BCD Packed BCD is a way of representing 2 BCD digits in an 8 bit value. Packed BCD value $25 0010 0101

Unpacked BCD value $02 $05 0000 0010 0000 0101

ASCII code value $32 $35 0011 0010 0011 0101

ASCII to BCD Conversion As a practical exercise consider the circumstance where 2 ASCII values are to be converted to Packed BCD. Key 4 7

ASCII value 34 37

Binary value 0011 0100 0011 0111

Unpacked BCD value 0000 0100 0000 0111

The following Assembler code converts ASCII to packed BCD

packBCD:

ldaa anda lsla lsla lsla lsla staa ldaa anda oraa staa swi end

org $2100 ds 01 org $2000 #’4’ #$0F

packBCD #’7’ #$0F packBCD packBCD

; reserve memory for packed BCD result

;load A with ASCII code for ‘4’ => 34 ;convert to unpacked BCD 34 => 04 ;shift lower nibble to upper nibble ;shift left 4 times 04 => 40

;store upper nibble ;load A with ASCII code for ‘7’ => 37 ;convert to unpacked BCD 37 => 07 ;combine lower and upper nibbles ;store packed BCD result of 47

Packed BCD value 0100 0111

An Alternate Programming Solution

packBCD: ldaa anda tab ldab mul ldaa anda aba staa swi end

org $2100 ds 01 org $2000 #’4’ #$0F #$10 #’7’ #$0F packBCD

; reserve memory for packed BCD result ;load A with ASCII code for ‘4’ => 34 ;convert to unpacked BCD 34 => 04 ;transfer A to B ;shift left 4 times by multiplying by $10 or 16 ; 04 => 40 ;load A with ASCII code for ‘7’ => 37 ;convert to unpacked BCD 37 => 07 ;combine lower and upper nibbles – 40+07=47 ;store packed BCD result of 47

Packed BCD to ASCII Conversion The following program converts a Packed value to two ASCII coded bytes.

High_Byte: Low_Byte: PackBCD:

ldaa anda lsra lsra lsra lsra oraa staa ldaa anda oraa staa swi end

org $2100 ds 01 ds 01 equ $39 org $2000 #PackBCD #$F0

#$30 High_Byte #PackBCD #$0F #$30 Low_Byte

; reserve memory for upper byte result ; reserve memory for lower byte result ;sample data to unpack

;load A with $39 – packed BCD ;mask the lower nibble 39 => 30 ;shift right 4 times 30 => 03

;convert to ASCII 03 => 33 - adda #$30 also works ;store in memory ;load A with $39 – packed BCD ;mask the upper nibble 39 => 09 ;convert to ASCII 09 => 39 - adda #$30 also works ;store in memory