CCS C Compiler Manual PCB / PCM / PCH

+ 0.3 0.4 0.6 51.3
- 0.3 0.4 0.6 52.3
* 0.4 3.2 22.2 35.8
/ 11.3 32 106.6 144.9
exp() * * * 510.4
ln() * * * 644.8
sin() * * * 698.7
Instead of 800, the compiler calls 0. Why?
The PIC® ROM address field in opcodes is 8-10 Bits depending on the chip and specific
opcode. The rest of the address bits come from other sources. For example, on the 174 chip to
call address 800 from code in the first page you will see:
BSF 0A,3
CALL 0
The call 0 is actually 800H since Bit 11 of the address (Bit 3 of PCLATH, Reg 0A) has been set.
Instead of A0, the compiler is using register 20. Why?
The PIC® RAM address field in opcodes is 5-7 bits long, depending on the chip. The rest of the
address field comes from the status register. For example, on the 74 chip to load A0 into W you
will see:
BSF 3,5
MOVFW 20
Note that the BSF may not be immediately before the access since the compiler optimizes out
the redundant bank switches.
What can be done about an OUT OF RAM error?
The compiler makes every effort to optimize usage of RAM. Understanding the RAM allocation
can be a help in designing the program structure. The best re-use of RAM is accomplished
when local variables are used with lots of functions. RAM is re-used between functions not
active at the same time. See the NOT ENOUGH RAM error message in this manual for a more
detailed example.
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