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5-3
PROGRAMMING
Figure 5-1. Word and Double-word Storage in Big Endien Form
Instructions in the MCS 51 architecture use 80H–FFH as addresses for both memory locations
and SFRs, because memory locations are addressed only indirectly and SFR locations are ad-
dressed only directly. For compatibility, software tools for MCS 251 controllers recognize this
notation for instructions in the MCS 51 architecture. No change is necessary in any code written
for MCS 51 controllers.
For new instructions in the MCS 251 architecture, the memory region prefixes (00:, 01, ..., FF:)
and the SFR prefix (S:) are required. Also, software tools for the MCS 251 architecture permit
00: to be used for memory addresses 00H–FFH and permit the prefix S: to be used for SFR ad-
dresses in instructions in the MCS 51 architecture.
Table 5-2. Notation for Byte Registers, Word Registers, and Dword Registers
Register
Type
Register
Symbol
Destination
Register
Source
Register
Register Range
Byte
Ri — — R0, R1
Rn — — R0–R7
Rm Rmd Rms R0–R15
Word WRj WRjd WRjs WR0, WR2, WR4, ..., WR30
Dword DRk DRkd DRks DR0, DR4, DR8, ..., DR28, DR56, DR60
MOV WR0,#A3B6H
MOV 00:0201H,WR0
MOV DR4,#0000C4D7H
A3H
B6H
2
3
WR0
4
5
6
0
1
7
00H 00H
C4H
D7H
DR4
Register File
Memory
Contents of register file and memory after execution
A4242-01
200H
201H
202H
203H
A3H
B6H