Texas Instruments MSC1210 - Manual (Page 59)

External Direct Addressing
5-5
Addressing Modes
5.5 External Direct Addressing
External memory is accessed using a suite of instructions that use external
direct addressing. It is referred to as external direct because it appears to be
direct addressing, but it is used to access external memory rather than internal
memory.
There are only two commands that use external direct addressing mode:
MOVX A,@DPTR
MOVX @DPTR,A
As you can see, both commands use DPTR. In these instructions, DPTR must
first be loaded with the address of external memory that you wish to read or
write. Once DPTR holds the correct external memory address, the first com-
mand moves the contents of that external memory address into the accumula-
tor. For example, if you want to read the contents of external RAM address
1516
H
, execute the instructions:
MOV DPTR,#1516h ;Select the external address to read
MOVX A,@DPTR ;Move the contents of external RAM into
;accumulator
The second command does the opposite: it allows you to write the value of the
accumulator to the external memory address pointed to by DPTR. For exam-
ple, if you want to write the contents of the accumulator to external RAM ad-
dress 1516
H
, execute the instructions:
MOV DPTR,#1516h ;Select the external address to read
MOVX @DPTR,A ;Move the contents of external RAM into
;accumulator
MOVX to data flash memory writes to the data flash memory location. To clear
the flash content, page erase is needed.