Bit Functions Introduction

All bit functions use absolute bit addressing.

The absolute bit address is the address of the bit within the PIC data space.

Each byte (RAM or SFR location) within the PIC data space has a unique address.

The relative address of a bit within a data byte is between 0 and 7 and is relative to the byte.

BYTE		relative BIT address
63		7	6	5	4	3	2	1	0
64		7	6	5	4	3	2	1	0
65		7	6	5	4	3	2	1	0
66		7	6	5	4	3	2	1	0

The absolute address of a bit within the entire data space is between 0 and 65535 The absolute bit address is calculated as ((N * 8) + M) where N is the byte address of the byte within the PIC data space and M is the relative bit address of the bit within byte N

BYTE		absolute BIT address
63		511	510	509	508	507	506	505	504
64		519	518	517	516	515	514	513	512
65		527	526	525	524	523	522	521	520
66		535	534	533	532	531	530	529	528

NOTE: XCSB can accomodate absolute bit addresses upto 32 bit but given that currently no PICs are available with more than 8K of RAM the user is advised to use 16 bit variables to manipulate absolute bit addresses.

NOTE2: XCSB can efficiently convert constant absolute bit addresses to single PIC instructions.

The expression ((&PORTA * 8) + 3) has no runtime overhead when used as an argument to the set_bit, reset_bit and test_bit functions

e.g.

set_bit(((&PORTA * 8) + 3))

generates only 1 PIC instruction

if test_bit(((&PORTA * 8) + 3)) != 0 then

generates only 2 PIC instructions

constant absolute bit addresses can be carried efficiently through function calls if the called function is defined as inline. If the called function is not defined as inline then there is a runtime overhead invloved in accessing the byte where the bit is located then the bit itself. The user cannot define functions as inline within the LITE edition of XCSB. This is a restricted feature. However the set_bit, reset_bit and test_bit library functions defined within bit_lib.bas are not restricted and are defined as inline functions.