UMBC CMSC 391 -- Programming Microcontrollers  

Direct Addressing Mode

This mode deals with the internal memory only, 128 bytes. It also includes all of the SFRs. In your program, you can use the SFR name or its address, interchangably. It is better style, much better, to use the SFR name.

Moving data from a direct address to itself is not predicable and could lead to errors.

Equation Table Of SFR Name To Address
Numeric Order
addressSFR name
080 P0
081 SP
082 DPL
083 DPH
087 PCON
088 TCON
089 TMOD
08A TL0
08B TL1
08C TH0
08D TH1
090 P1
098 SCON
099 SBUF
0A0 P2
0A8 IE
0B0 P3
0B8 IP
0D0 PSW
0E0 A
0F0 B

Note:Where there are gaps, there is no memory and those addresses do not work as expected!

Note:I have put a leading zero on all addresses. I only have to do it for addresses in the range of 0A0h to 0FFh, but there is no problem if I put a zero in front of all of them for consistence sake.

Equation Table Of SFR Name To Address
Alphabetic Order
addressSFR name
A 0E0
B 0F0
DPH 083
DPL 082
IE 0A8
IP 0B8
P0 080
P1 090
P2 0A0
P3 0B0
PCON 087
PSW 0D0
SBUF 099
SCON 098
SP 081
TCON 088
TMOD 089
TH0 08C
TH1 08D
TL0 08A
TL1 08B

Examples

2000:                   .org    0x2000
                   
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                   begin:
                           ;;A is at 0E0h, so are they
                           ;;  interchangable?
2000: 75 E0 0F             mov     0E0h, #15 ; 3 bytes, 2 cycles
2003: 74 0F                mov     A, #15    ; 2 bytes, 1 cycle
                   
                           ;; What about B?
2005: 75 F0 0F             mov     0F0h, #15
2008: 75 F0 0F             mov     B, #15
                   
                           ;; Serial I/O buffer
200B: E5 99                mov     A, SBUF
200D: 85 20 99             mov     SBUF, #32   ; Output a blank character
2010: 85 41 99             mov     SBUF, 'A'
2013: 22                   ret
                   
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©2004, Gary L. Burt