Ax51 User's Guide

BIT

Memory locations in the memory class BIT are addressed with the bit instructions of the 8051. The Special Function Registers (SFR) that are located in bit-addressable memory locations can be addressed with bit instructions. Bit-addressable SFR locations are: 80H, 88H, 90H, 98H, 0A0H, 0A8H, 0B0H, 0B8H, 0C0H, 0C8H, 0D0H, 0D8H, 0E0H, 0E8H, 0F0H, and 0F8H.

Example for all 8051 variants

?BI?mybits SEGMENT BIT          ; define a SEGMENT of class BIT
           RSEG    ?BI?mybits
FLAG:      DBIT    1            ; reserve 1 Bit in BIT space
P1         DATA    90H          ; 8051 SFR PORT1
GREEN_LED  BIT     P1.2         ; GREEN LED on I/O PORT P1.2

?PR?myprog SEGMENT CODE         ; define a segment for program code
           RSEG    ?PR?myprog
           SETB    GREEN_LED    ; P1.2 = 1
           JB      FLAG,is_on   ; direct memory access to DATA
           SETB    FLAG
           CLR     ACC.5        ; reset bit 5 in register A
            :
is_on:     CLR     FLAG
           CLR     GREEN_LED     ; P1.2 = 0