DA Example

DA Example

Directives highlighted in this example are:

• da

Program Functional Description

This example shows the usefulnes of directive da in storing a character string in the program memory of 14-bit architecture devices. This directive generates a packed 14-bit number representing two 7-bit ASCII characters.

Commented Code Listing

list p=16f877 ;Select the device.

#include <p16f877.inc> ;Include standard header file

;for the selected device.

ORG 0000 ;The following code will be

;programmed in reset address 0.

goto start ;Jump to an address labelled

;'start'.

start ;Write your main program here to

;display the string given in

;'Ch_stng'.

goto $

ORG 1000 ;Store the string starting from

;1000H.

Ch_stng da "PICmicro"

Sngl_ch da "A" ;7-bit ASCII equivalents of 'A'

;and a NULL charater will be packed

;in a 14-bit number.

da 0xff55 ;Places 3f55 in program memory.

;No packing.

end

Additional Comments

Determining 14-Bit Numbers

For the following statement:

Ch_stng da "PICmicro"

directive da produces four 14-bit numbers: 2849, 21ED, 34E3 and 396F representing the ASCII equivalent of PI, Cm, ic and ro.

To see how the 14-bit numbers are determined, let's look at the ASCII values of P and I, which are 50h(01010000) and 49h(01001001) respectively. Each is presented in 7-bit as (0)1010000 and (0)1001001 respectively. The packed 14-bit number is 101000 01001001, which is stored as (00)101000 01001001 or 2849.