Page 215 - Programming Microcontrollers in C

200 Chapter 4 Small 8-Bit Systems

0151 0F 09 FD BRCLR 7,$09,$0151
while(AD_CTST.COCO==0);
0154 B6 08 LDA $08 return AD_DATA;
0156 81 RTS
}

0157 BF 5A STX $5A
0159 B7 5B STA $5B
015B 4F CLRA
015C 5F CLRX
015D 5C INCX
015E 38 5B LSL $5B
0160 49 ROLA
0161 B0 5A SUB $5A
0163 24 03 BCC $0168
0165 BB 5A ADD $5A
0167 99 SEC
0168 59 ROLX
0169 24 F3 BCC $015E
016B 53 COMX
016C 81 RTS

1FFE 01 00
The assembly code to execute this single line of C code is found in
the address range $10c to $11b, or 17 bytes of code. However,
there is a call to the double precision add routine at address $115.
This routine occupies the address range $157 to $16c, or 22 addi­
tional bytes of code hidden from the main routine. Therefore, this
requires 37 bytes of code for its execution.
The next approach is to avoid the overflow problem by dividing
by 2 each of the terms to be added prior to the addition. Division by
2 is accomplished by shifting each of the terms to the right by one
bit. This approach guarantees that there will be no overflow into the
higher byte because the most significant bit of each number will be 0
after the shift, and no binary addition can cause more than a 1-bit
overflow. Therefore, the sum will at most have its most significant
bit turned on. This number is then stored in the location
adc_data[j].