16-bit sum of data
Purpose:
To calculate the sum of a series of numbers. The length of the series given as a number of words (i.e. 16 bit quantities) is in the variable length at location $6000. The starting address of the series is contained in the long-word variable start at location $6002. The sum is to be stored in the variable total at location $6006. It is to be assumed that the sum is a 16-bit number so that you can ignore carries.
Note: the specification above says that the starting address is in the variable start at location $6002. This is, of course, short for saying that the 32-bit value is spread over the locations with addresses $6002, $6003, $6004 and $6005.
Sample Test Data:
input:
length: (6000) = 0003
(6002) = 00005000
(5000) = 2040
(5002) = 1c22
(5004) = 0242)
output:
total: (6006) = (5000) + (5002) + (5004)
= 2040 + 1c22 + 0242 = 3ea4
Source Code:
| data | equ | $6000 | |||
| program | equ | $4000 | |||
| org | data | ||||
| length | ds.w | 1 | ;number of data elements | ||
| start | ds.l | 1 | ;address of data elements | ||
| ds.w | 1 | ;sum of data elements | |||
| org | program | ||||
| movea.l | start, a0 | ;initialise pointer | |||
| moveq | #0, d0 | ;initialise sum to 0 | |||
| move.w | length, d1 | ;initialise element count | |||
| loop | add.w | (a0)+,d0 | ;sum next element | ||
| subq.w | #1, d1 | ;update element count | |||
| bne | loop | ;if count not zero goto loop | |||
| move.w | d0,total | ;store sum | |||