A251 User's Guide

DATA

Memory locations in the memory class DATA can be addressed with both direct and indirect memory accesses. Special Function Registers (SFR) of the A251 have addresses above 0x80 in the DATA memory class. SFR locations can only be addressed with direct memory accesses. An indirect memory access to SFRs is not supported in the A251 microcontrollers.

Example for all 251 variants

?DT?myvar  SEGMENT DATA         ; define a SEGMENT of class DATA
           RSEG    ?DT?myvar
VALUE:     DS      1            ; reserve 1 BYTE in DATA space

IO_PORT2   DATA    0A0H         ; special function register
VALUE2     DATA    20H          ; absolute memory location

?PR?myprog SEGMENT CODE         ; define a segment for program code
           RSEG    ?PR?myprog
           MOV     A,IO_PORT2   ; direct memory access to DATA
           ADD     A,VALUE
           MOV     VALUE2,A
           MOV     R1,#VALUE    ; load address of VALUE to R1
           ADD     A,@R1        ; indirect memory access to VALUE