1. Chapter 5 8051 Addressing Modes

2. Sections 5.1 Immediate and register addressing modes
5.2 Accessing memory using various address modes

3. Objective
程式中的資料可能是放在 Register 中，或在RAM 中某一位址上，或在 ROM 一塊特殊區域放置資料，或者是指令中直接給予定值。
設計 8051 IC 的人們，提供這些存取資料的方式。這些方式便叫作 Addressing Mode。
中文稱為“定址模式”：決定參數位址的模式
也許不同家的 Assembler 會有不同的指令寫法，但基本上 addressing mode 都是一樣的。

4. Section 5.1 Immediate and Register Addressing Modes

5. What is Addressing Mode
The CPU can access data in various ways.
The data could be in a register, or in memory（RAM or ROM）, or be provided as an immediate value.
These various ways of accessing data are called addressing mode.

6. Addressing Mode in the 8051 Five addressing mode in the 8051：
1. immediate
2. register
3. direct
4. register indirect
5. indexed
先不管這些名詞的意義, 先猜猜看有哪些方法可以存取記憶體 ROM 和 RAM.

7. Addressing Mode 1 1. immediate － the operand is a constant
MOV A,#01FH
2. register － the operand is in a register
MOV A,R0
3. direct － access the data in the RAM with address
MOV A,01FH
4. register indirect － the register holds the RAM address of the data
MOV
5. indexed － for on-chip ROM access
MOVC

8. Immediate Addressing Mode
The source operand is a constant.
When the instruction is assembled, the operand comes immediately after the opcode.
The immediate vale can be loaded into any of the registers.
The immediate data must be preceded by the pound sign, ‘＃’.
The immediate value is bounded by the size of register.
Please use the simulation tools to find the the machine code and the content of registers after execution.
See Tables 10 ＆11 (page 418).

9. Example of Immediate Mode（1/2）
MOV A,#25H ;A=25H
C 3E MOV R4,#62 ;A=62=3EH
MOV DPTR,#4521H
Instruction Opcodes in Table 11
Hex code Mnemonic Operands Byte
MOV A, #data
7C MOV R4, #data
MOV DPTR, #data
DPTR has high byte DPH (address 83) and low byte DPL (address 82)
However, 8051 CPU does not provide a special machine instructions for MOV DPH, #data or MOV DPL,#data,
Although 8051 Assembler provide the assembly instructions for DPH and DPL

10. Example of Immediate Mode（2/2）
MOV A,#25H ;A=25H
C 3E MOV R4,#62 ;A=62=3EH
MOV DPTR,#4521H
Instruction Opcodes in Table 10
Mnemonic Oscillator Period
MOV A, #data
MOV Rn, #data
MOV DPTR, #data
Rn - Register R7-R0 of the currently selected Register Bank. in ‘Table 10 page 418

11. EQU The EQU directive is used in the immediate addressing mode.
ORG 0H
COUNT EQU 30
C 1E MOV R4,#COUNT
MOV DPTR,#MYDATA
ORG 200H
D MYDATA DB "America"
END
Assembler will transfer the COUNT=30=1EH to the immediate value.
MYDATA is the address of “America”, point to “A”
Assembler will transfer the MYDATA to the address 200H.
ORG 0H
COUNT EQU 30
C 1E MOV R4,#COUNT ;R4=30
MOV DPTR,#MYDATA ;DPTR=200H
ORG 200H
D MYDATA DB "America"
END

12. Addressing Mode 2 1. immediate － the operand is a constant
MOV A,#01FH
2. register － the operand is in a register
MOV A,R0
3. direct － access the data in the RAM with address
MOV A,01FH
4. register indirect － the register holds the RAM address of the data
MOV
5. indexed － for on-chip ROM access
MOVC

13. Register Addressing Mode
Register addressing mode involves the use of registers to hold the data.
The source and destination registers must match in size.
The movement of data between Rn registers is not allowed. “MOV R4,R7” is illegal.
You can find that the opcode in register addressing mode is short！

14. Example of Register Mode（1/2）
E MOV A,R0
FA MOV R2,A
D ADD A,R5
Instruction Opcodes in Table 11
Hex code Mnemonic Operands Byte
E MOV A,R
FA MOV R2,A
2D ADD A,R

15. Example of Register Mode（2/2）
E MOV A,R0
FA MOV R2,A
D ADD A,R5
Instruction Opcodes in Table 10
Mnemonic Oscillator Period
MOV A, Rn
MOV Rn, A
ADD A, Rn

16. Section 5.2 Accessing Memory Using Various Address Modes

17. Addressing Mode 3 1. immediate － the operand is a constant
MOV A,#01FH
2. register － the operand is in a register
MOV A,R0
3. direct － access the data in the RAM with address
MOV A,01FH
4. register indirect － the register holds the RAM address of the data
MOV
5. indexed － for on-chip ROM access
MOVC

18. Direct Addressing Mode
There are 128 bytes of RAM in the 8051.
The RAM has been assigned address FH.
00-1FH：the register banks and stack
20-2FH：bit-addressable space to save single-bit data
30-7FH：scratch pad RAM
In direct addressing mode, the data is in a RAM memory location whose address is known, and this address is given as a part of the instruction.
If an number begins without a pound sign, ‘＃’, then Assembler think it as the RAM address.
It is most often used to access RAM location 20-7FH.
This is due to the fact that register bank locations are accessed by the register names of R0-R7 -- register mode.
There is no such name for other RAM locations. -- so use direct address mode.

19. Example of Direct Mode（1/2）
A MOV R0,40H
F MOV 56H,A
MOV DPTR,#4521
MOV DPH,#45H
5 000A MOV DPL,#21H
Instruction Opcodes Table 11
Hex code Mnemonic Operands Bytes
A MOV R0, data addr
F MOV data addr., A
MOV data addr., #data
“Direct” means “data address”.
DPTR has high byte DPH (address 83) and low byte DPL (address 82)

20. Example of Direct Mode（2/2）
A MOV R0,40H
F MOV 56H,A
MOV DPTR,#4521
MOV DPH,#45H
5 000A MOV DPL,#21H
Instruction Opcodes Table 10
Mnemonic Oscillator Period
MOV Rn, direct
MOV direct, A
MOV direct, #data
“Direct” means “data address”.
DPTR has high byte DPH (address 83) and low byte DPL (address 82)

21. Register Bank（1/2）
If we use register bank 0, then the following instructions 2＆3 do the same works：
C 64 MOV R4,#100
E5 04 MOV A,4 ;direct mode
EC MOV A,R4 ;register mode
Initially, the 8051 uses the register bank 0.
R4 has RAM address 04H.

22. Register Bank（2/2）
If we use register bank 1, then the following instructions 3＆4 do the different works：
D2 D3 SETB RS0 ;RS0=1
C 64 MOV R4,#100
E5 04 MOV A,4 ;A=0
EC MOV A,R4 ;A=100=64H
RS1=PSW.4=0 ＆ RS0=PSW.3=1  register bank 0
Initially, the content of RAM is 00H.
R4 has RAM address 0CH.
RAM 0CH has the value 100.

23. SFR（Special Function Register）
There are many special functions registers in the We call them SFR.
Example：A, B, PSW, and DPTR
The 8051 Assembler provides that the SFR can be accessed by their name or by their addresses.
See Table 5-1 for SFR addresses
The SFR have addresses between 80H and FFH.
Not all the address space of 80 to FF is used by the SFR.
The addresses 00-0FH are addresses of RAM memory inside the So SFR needs to use the addresses 80H to FFH.
The addresses 80H to FFH is not a really RAM.
The unused locations 80H to FFH are reserved and must not be used by the 8051 programmer.
We can access SFR by direct addressing mode.

24. Table 5-1: Special Function Register (SFR) Addresses（1/2）
Symbol
Name
Address
ACC*
Accumulator
0E0H B*
B register
0F0H
PSW*
Program status word
0D0H SP
Stack pointer
81H
DOTR
Data pointer 2 bytes
DPL
Low byte
82H
DPH
High byte
83H
P0*
Port 0
80H
P1*
Port 1
90H
P2*
Port 2
0A0H
P3*
Port 3
0B0H
IP*
Interrupt priority control
0B8H
IE*
Interrupt enable control
0A8H
TMOD
Timer/counter mode control
89H

25. Table 5-1: Special Function Register (SFR) Addresses （2/2）
Symbol
Name
Address
TCON*
Timer/counter control
88H
T2CON*
Timer/counter 2 control
0C8H
T2MOD
Timer/counter mode control
0C9H
TH0
Timer/counter 0 high byte
8CH
TL0
Timer/counter 0 low byte
8AH
TH1
Timer/counter 1 high byte
8DH
TL1
Timer/counter 1 low byte
8BH
TH2
Timer/counter 2 high byte
0CDH
TL2
Timer/counter 2 low byte
0CCH
RCAP2H
T/C 2 capture register high byte
0CBH
RCAP2L
T/C 2 capture register low byte
0CAH
SCON*
Serial control
98H
SBUF
Serial data buffer
99H
PCON
Power control
87H
*bit addressable (discussed further in Chapter 8)

26. ACC and Its Address ACC has SFR address 0E0H.
E0 55 MOV 0E0H,#55H
MOV A,#55H
D2 E1 SETB A.1
Compare their code size and execution time.
“ACC＊”, ＊ means this register is bit addressable. You can access each bit of ACC independently.
E0 55 MOV 0E0H,#55H : 2 bytes 2 MCs
MOV A,#55H : 2 bytes 1 MC
For address 0E0H, if the instruction is for a bit “SETB”, then it refers to A.0
if the instruction is for a byte “MOV”, then it refers to A.
SFC addr. 0E7 0E E5 0E E3 0E2 0E1 0E0
ACC
A.7
A.6
A.5
A.4
A.3
A.2
A.1
A.0

27. Example 5-1 Write code to send 55H to ports P1 and P2, using
(a) their names
(b) their addresses.
Solution:
(a) MOV A,#55H ;A=55H
MOV P1,A ;P1=55H
MOV P2,A ;P2=55H
(b) From Table 5-1, P1 address = 80H; P2 address = A0H
MOV A,#55H ;A=55H
MOV 80H,A ;P1=55H
MOV 0A0H,A ;P2=55H

28. Stack Another major use of direct addressing mode is the stack.
In the 8051 family, only direct addressing mode is allowed for pushing onto the stack.

29. Example 5-2（1/2）
Show the code to push R5, R6, and A onto the stack and then pop
them back them into R2, R3, and B.
We want：B = A, R2 = R6, and R3 = R5.
Solution:
PUSH ;push R5 onto stack
PUSH ;push R6 onto stack
PUSH 0E0H ;push register A onto stack
POP 0F0H ;pop top of stack into register B
POP ;pop top of stack into R2
POP ;pop top of stack into R3
C PUSH R5
C PUSH R6
C0 E0 PUSH A
C PUSH 05
C PUSH 06
C0 E0 PUSH 0E0H

30. Example 5-2（1/2）
Different assembler provide different instruction for the stack.
In our simulation tools, they are the same：
C PUSH R5
C PUSH R6
C0 E0 PUSH A
C PUSH 05
C PUSH 06
C0 E0 PUSH 0E0H

31. Addressing Mode 4 1. immediate － the operand is a constant
MOV A,#01FH
2. register － the operand is in a register
MOV A,R0
3. direct － access the data in the RAM with address
MOV A,01FH
4. register indirect － the register holds the RAM address of the data
MOV
5. indexed － for on-chip ROM access
MOVC

32. Register Indirect Addressing Mode
In the register indirect addressing mode, a register is used as a pointer to the data.
That is, this register holds the RAM address of the data.
Only registers R0 and R1 can be used to hold the address of an operand located in RA.
Usually, R0 and R1 are denoted by Ri.
When R0 and R1 hold the addresses of RAM locations, they must be preceded by the sign.
So it is called “indirect”.
If you forget sign, 8051 Assembler will use the content of Ri.

33. Example of Register Indirect Mode（1/2）
MOV 20H,#100
MOV R0,#20H
E MOV
DPTR has high byte DPH (address 83) and low byte DPL (address 82)
However, 8051 CPU does not provide a special machine instructions for MOV DPH, #data or MOV DPL,#data,
Although 8051 Assembler provide the assembly instructions for DPH and DPL
RAM
1. put 64H to addr. 20H
2. let R0 be the data address
1E F :
R H
3. copy the content in addr. R0=20H to A
A H

34. Example of Register Indirect Mode（2/2）
F0 80 MOV B,#080H
MOV R1,#31H
A7 F0
DPTR has high byte DPH (address 83) and low byte DPL (address 82)
However, 8051 CPU does not provide a special machine instructions for MOV DPH, #data or MOV DPL,#data,
Although 8051 Assembler provide the assembly instructions for DPH and DPL
RAM
3. copy B to the RAM location with addr. R1=31H
2F :
2. let R0 be the data address
R H
1. let B=80H
B H

35. Example 5-3 (1/3)
Write a program to copy the value 55H into RAM memory locations 40H to 45H using
(a) direct addressing mode,
(b) register indirect addressing mode without a loop,
(c) with a loop.
Solution of (a) :
MOV A,#55H
MOV 40H,A
MOV 41H,A
MOV 42H,A
MOV 43H,A
MOV 44H,A
RAM
copy A to the RAM location of addr. 43H
A H

36. Example 5-3 (2/3)
Solution of (b) register indirect addressing mode without a loop
MOV A,#55H ;load A with value 55H
MOV R0,#40H ;load the pointer. R0=40H
;copy A to RAM location where R0
; points to
INC R ;increment pointer. Now R0=41H
INC R ;R0=42H
INC R ;R0=43H
INC R0
RAM
R H
A H

37. Example 5-3 (3/3) Solution of (c) with a loop: MOV A,#55H ;A=55H
MOV R0,#40H ;load pointer. R0=40H,
MOV R2,#05H ;load counter, R2=5
AGAIN: MOV @R0,A ;copy 55 to RAM location
; R0 points to
INC R ;increment R0 pointer
DJNZ R2,AGAIN ;loop until counter = 0

38. Advantage of Register Indirect Addressing Mode
One of the advantages of register indirect addressing mode is that it makes accessing data dynamic rather than static.
Solution (c) in Example 5-3 is the most efficient and is possible only because of register indirect addressing mode.
Looping is not possible in direct addressing mode.
See Examples 5-4, 5-5, too.
Their use is limited to accessing any information in the internal RAM.
Use looping is more flexible too.
If we need to access externally connected RAM or on-chip ROM, we need a 16-bit pointer. In such cases, the DPTR register is used. That is, indexed addressing mode.

39. Example 5-4
Write a program to clear 16 RAM locations starting at RAM address
60H.
Solution:
CLR A ;A=0
MOV R1,#60H ;load pointer. R1=60H
MOV R7,#16 ;load counter, R7=10H
AGAIN: MOV @R1,A ;clear RAM location R1
; points to
INC R ;increment R1 pointer
DJNZ R7,AGAIN ;loop until counter = 0

40. Example 5-5
Write a program to copy a block of 10 bytes of data from RAM locations starting at 35H to RAM locations starting at 60H.
Solution:
MOV R0,#35H ;source pointer
MOV R1,#60H ;destination pointer
MOV R3,#10 ;counter
BACK: MOV ;get a byte from source
MOV @R1,A ;copy it to destination
INC R ;increment source pointer
INC R ;increment destination
; pointer
DJNZ R3,BACK ;keep doing it 10 times

41. Addressing Mode 5 1. immediate － the operand is a constant
MOV A,#01FH
2. register － the operand is in a register
MOV A,R0
3. direct － access the data in the RAM with address
MOV A,01FH
4. register indirect － the register holds the RAM address of the data
MOV
5. indexed － for on-chip ROM access
MOVC

42. Indexed Addressing Mode
Indexed addressing mode is widely used in accessing data elements of look-up table entries located in the program ROM space of the 8051.
A look-up table is a ROM block where the data is given previously (then you can access it frequently).
The instruction used for this purpose is MOVC.
DPTR can be used to access memory externally connected to the See Chapter 14.
Another register used in indexed addressing mode is the PC. See Appendix A.

43. MOVC Copy the source operand to the destination operand.
MOVC A, @A+DPTR
The “C” means code (program code in on-chip ROM).
A+DPTR is the address of the data element stored in on-chop ROM.
Put the ROM value to A.

44. Example of MOVC Register Indexed addressing Mode：
MOV DPTR,#MYDATA
E CLR A
MOVC
F MOV R0,A
FE HERE: SJMP HERE
MYDATA: DB "USA“
DPTR=#MYDATA=0008H
A+DPTR=0008H
ROM E4 :
A 41
A H

45. Example 5-6 (1/2)
In this program, assume that the word “USA” is burned into ROM
locations starting at 200H, and that the program is burned into
ROM locations starting at 0. Analyze how the program works and
state where “USA” is stored after this program is run.
Solution:
ROM E4 :
DPTR H H
A+DPTR= 0200H
U S A
R H
A H
R H
R H

46. Example 5-6 (2/2) ORG 0000H ;burn into ROM from 0
MOV DPTR,#200H ;DPTR=200H
CLR A ;clear A(A=0)
MOVC ;get the char space
MOV R0,A ;save it in R0
INC DPTR ;DPTR=201
MOVC ;get the next char
MOV R1,A ;save it in R1
INC DPTR ;DPTR=202
MOV R2,A ;save it in R2
HERE:SJMP HERE ;stay here
ORG 200H
MYDATA: DB “USA”
END ;end of program

47. Example 5-7 (1/2)
Assuming that ROM space starting at 250H contains “America”, write a program to transfer the bytes into RAM locations starting at 40H.
Solution of (a) This method uses a counter：
ORG 0000
MOV DPTR,#MYDATA ;Initialization
MOV R0,#40H
MOV R2,#7
BACK: CLR A
MOVC
MOV @R0,A
INC DPTR
INC R0
DJNZ R2,BACK
HERE: SJMP HERE
ORG 250H
MYDATA: DB “AMERICA”
END
ROM
RAM : D
D E
AME R I C A N
A 41
R0 40
DPTR

48. Example 5-7 (2/2)
Solution of (b) This method uses null char for end of string：
ORG 0000
MOV DPTR,#MYDATA
MOV R0,#40H ;No “MOV R2,#7”
BACK: CLR A
MOVC
JZ HERE ;if A=0
MOV @R0,A ;leave the block
INC DPTR
INC R0
SJMP BACK
HERE: SJMP HERE
ORG 250H
MYDATA: DB “AMERICA”,0 ;notice null char
;for end of string
END

49. Example 5-8
Write a program to get the x value from P1 and send x2 to P2,
continuously.
Solution:
ORG 0
MOV DPTR,#XSQR_TABLE
MOV A,#0FFH
MOV P1,A ;P1 as INPUT PORT
BACK: MOV A,P ;GET X
MOVC ;Count the addr.
MOV P2,A ;Issue it to P2
SJMP BACK
ORG 300H
XSQR_TABLE:
DB 0,1,4,9,16,25,36,49,64,81
END
Please trace the program by set A=0, then A=1, then A=2,....

50. Example 5-9 Answer the following questions for Example 5-8.
(a) Indicate the content of ROM locations H.
(b) At what ROM location is the square of 6, and what value should
be there?
(c) Assume that P1 has a value of 9: what value is at P2 (in binary)?
Solution:
(a) All values are in hex.
300 = (00) 301 = (01) 302 = (04) 303 = (09)
304 = (10) 4×4=16=10H
305 = (19) 5×5=25=19H
306 = (24) 6×6=36=24H
307 = (31) 308 = (40) 309 = (51)
(b) ROM Addr.=306H; the value 24H=36
(c) P2 = B=51H=81 in decimal.

51. You are able to
List the 5 addressing modes of the 8051 microcontroller
Contrast and compare the addressing modes
Code 8051 Assembly language instructions using each addressing mode
List the SFR（special function registers）address
Discuss how to access the SFR
Manipulate the stack using direct addressing mode
Code 8051 instructions to manipulate a look-up table

52. Homework Chapter 5 Problems：2,3,8,11,12,13 Note:
Please write and compile the program of Problems 8,11,12,13.