1. Chapter 8 Single-bit Instructions and Programming

2. Objective
8051 有一個很特殊且 powerful 的功能：8051 允許 programmer 每次只存取一個 bit (而非一個 byte) 的方式，使用在 RAM 中或 register 的資料。這樣的方式，稱為 bit-addressable。
只有在RAM 中某些特定的位址，或是某些特定的 registers，提供 bit-addressable 的功能。
相對的， 8051 也提供一些特殊的指令，專門用來對一個 bit 做存取或運算。
這章節中就是要去了解這些關於 bit-addressable 指令的用法。

3. Sections 8.1 Single-bit Instruction Programming
8.2 Single-bit Operations with CY
8.3 Reading Input Pins vs. Port Latch

4. Section 8.1 Single-bit Instruction Programming

5. Bit-addressability (1/2)
In most microprocessors, data is accessed in byte-sized chunks.
However, in many applications we need to change one bit, for example, to turn on or off a device.
The bit-addressability of the 8051
The ability to access data in single bit instead of the whole byte.
Make 8051 become one of the most powerful 8-bit microprocessors.

6. Bit-addressability (2/2)
Which portions of the microprocessor, I/O ports, registers, RAM, or ROM are bit-addressable?
ROM holds program code for executions. There is no need for bit-addressability.
I/O: all ports are bit-addressable
Register: some of them are bit-addressable
RAM: bit-addressable RAM (20H-3FH)

7. I/O Ports and Bit-addressability
We can access either the entire 8 bits or any single bit without altering the rest.
Example:
SETB P2.3 ;set pin 3 of P2 to high (1)
CLR P1.0 ;clear pin 0 of P1(0)
Remember that D0 is the LSB and D7 is the MSB.

8. Example 8-1 (a-1) Write the following program.
Create a square wave of 50% duty cycle on bit 0 of port 1.
Solution:
The 50% duty cycle means that the period of “on” state is 50% in the period of the whole pulse (“on” state pluses “off” state).
That is, the “on” and “off” state have the same length.
8051
P1.0
on state high portion
off state low portion
50%
50%
whole pulse

9. Example 8-1 (a-2)
Create a square wave of 50% duty cycle on bit 0 of port 1.
HERE:SETB P1.0 ;set to high bit 0 of port 1
LCALL DELAY ;call the delay subroutine
CLR P1.0 ;P1.0=0
LCALL DELAY
SJMP HERE
Another way to write the above program is:
HERE:CPL P1.0 ;complement bit 0 of port 1

10. Example 8-1 (b-1) Write the following programs.
(b) Create a square wave of 66% duty cycle on bit 3 of port 1.
Solution:
The 66% duty cycle means that the period of “on” state is 66% in the period of the whole pulse.
That is the “on” state is twice the “off” state.
8051
P1.3
66%
34%

11. Example 8-1 (b-2)
(b) Create a square wave of 66% duty cycle on bit 3 of port 1.
BACK:SETB P1.3 ;set port 1 bit 3 high
LCALL DELAY
LACLL DELAY
CLR P1.3 ;clear bit 2 of port 1
SJMP BACK

12. Bit Address of I/O Ports
Each port has its byte address.
Ex: P0 has address 80H.
Each pin of port has its bit address.
Ex: pin 7 of port 0 has bit address 87H
Note that pin 0 of port 0 has address 80H. Your instruction decides that the operand is bit or byte.
The bit address of ports
See Table 8-2 
See Figure 8-1 or Figure A-1 for SFR RAM address 

13. Table 8-2: Single-Bit Addressability of Ports
Port’s Bit
P0.0
P1.0
P2.0
P3.0 D0
P0.1
P1.1
P2.1
P3.1 D1
P0.2
P1.2
P2.2
P3.2 D2
P0.3
P1.3
P2.3
P3.3 D3
P0.4
P1.4
P2.4
P3.4 D4
P0.5
P1.5
P2.5
P3.5 D5
P0.6
P1.6
P2.6
P3.6 D6
P0.7
P1.7
P2.7
P3.7 D7 

14. Figure 8-1. SFR RAM Address (Byte and Bit) (1/2)
Special Function Registers
Byte address
Bit address
F7 F6 F5 F4 F3 F2 F1 F0
E7 E6 E5 E4 E3 E2 E1 E0
D7 D6 D5 D4 D3 D2 D1 D0
A7 A6 A5 A4 A3 A2 A1 A0
AF AC AB AA A9 A8
B7 B6 B5 B4 B3 B2 B1 B0
BC BB BA B9 B8 FF F0 E0 D0 B8 B0 A8 A0 B
ACC
PSW IP P3 IE P2

15. Figure 8-1. SFR RAM Address (Byte and Bit) (2/2)
Special Function Registers
9F 9E 9D 9C 9B 9A
D7 D6 D5 D4 D3 D2 D1 D0
A7 A6 A5 A4 A3 A2 A1 A0
not bit addressable 99 98 90 8D 8C 8B 8A 89
SBUF
SCON
TH1
TL0 P1
TMOD
DPH 88 87 83 82 81 80
TH0
TL1
TCON
PCON
DPL SP P0 

16. Example 8-2
For each of the following instructions, state which bit of which SFR will be affected. Use Figure 8-1.
(a) SETB 86H (b) CLR 87H (c) SETB 92H
(d) SETB 0A7H (e) CLR 0F2H (f) SETB 0E7H
Solution:
(a) SETB 86H is for SETB P0.6
(b) CLR 87H is for CLR P0.7
(c) SETB 92H is for SETB P1.2
(d) SETB 0A7H is for SETB P2.7
(e) CLR 0F2H is for CLR D2 of register B
(f) SETB 0E7H is for SETB ACC.7 (D7 of register A)

17. Checking an Input Bit
To monitor a bit and make a decision depending on where it is 0 or 1
JB bit,target ; jump if bit set
HERE: JB P2.1,HERE ;jump to HERE if P2.1=1
JNB bit,target ; jump if bit not set
HERE: JNB P2.1,HERE ;jump to HERE if P2.1=0
See Example 8-3

18. Example 8-3
Assume that bit P2.3 is an input and represents the condition of an
oven. If it goes high, it means that the oven is hot.
Monitor the bit continuously.
Whenever it goes high, send a high-to-low pulse to port P1.5 to turn on a buzzer.
Solution:
HERE:JNB P2.3,HERE ;keep monitoring for high
SETB P ;
CLR P ;

19. Registers and Bit-Addressability
Only registers B, A, PSW, IP, IE, SCON, TCON are bit-addressable.
See Figure 8-1 
Examples 8-4 and 8-5 use JNB, JB to monitor one bit.
Examples 8-6 and 8-7 control bits of PSW.

20. Example 8-4
Write a program to see if the accumulator contains an even number.
If so, divide it by 2.
If not, make it even and then divide it by 2.
Solution:
MOV B,# ;B=2
JNB ACC.0,YES ;If A=0, jump to YES
INC A ;it is odd, make it even
YES:DIV AB ;A/B

21. Example 8-5
Write a program to see if bits 0 and 5 of register B are 1. If they are
not, make them so and save it in R0.
Solution:
Use bit address of B.0 and B.5:
JB 0F0H,NEXT_1 ;jump if B.0=1
SETB 0F0H ;
NEXT_1: JB 0F5H,NEXT_2 ;jump if B.5=1
SETB 0F5H ;
NEXT_2: MOV R0,B ;save to R0

22. Example 8-6 Write a program to save the accumulator in R7 of bank 2.
Solution:
CLR PSW.3 ; Register bank 2
SETB PSW.4
MOV R7,A CY AC --
RS1
RS0 OV P
PSW
RS1
RS0
Register Bank
Address
00H – 07H 1
08H – 0FH 2
10H – 17H 3
18H – 1FH

23. Example 8-7
While there are instructions such as JNC and JC to check the carry flag bit (CY), there are no such instructions for the overflow flag bit (OV).
How would you write code to check OV?
Solution:
The OV flag is PSW.2 of the PSW register.
We can use the following instruction to check the OV flag.
JB PSW.2,TARGET ;jump if OV=1

24. Bit-addressable RAM The bit-addressable RAM locations are 20H to 2FH.
Only 16 bytes of RAM are bit-addressable.
16 * 8 bits = 128 bits (in decimal) = 80H bits (in hex)
They are addressed as 00 to 7FH
Note that the bit addresses 80H to F7H belong to SFR.
See Figure 8-3

25. Figure 8-3. 128 Bytes of Internal RAM
Byte address 7F
General purpose RAM 30
Bit-addressable locations 2F 7F 7E 7D 7C 7B 7A 79 78 77 76 75 74 73 72 71 70 6F 6E 6D 6C 6B 6A 69 68 67 66 65 64 63 62 61 60 5F 5E 5D 5C 5B 5A 59 58 57 56 55 54 53 52 51 50 4F 4E 4D 4C 4B 4A 49 48 47 46 45 44 43 42 41 40 3F 3E 3D 3C 3B 3A 39 38 37 36 35 34 33 32 31 30 2F 2E 2D 2C 2B 2A 29 28 27 26 25 24 23 22 21 20 1F 1E 1D 1C 1B 1A 19 18 17 16 15 14 13 12 11 10 0F 0E 0D 0C 0B 0A 09 08 07 06 05 04 03 02 01 00 2E 2D 2C 2B 2A 29 28 27 26 25 24 23 22 21 20 1F
Bank 3 18 17
Bank 2 10 0F
Bank 1 08 07
Default register bank for R0 - R7 00

26. Example 8-8 Find out to which byte each of the following bits belongs.
Give the address of the RAM byte in hex.
(a)SETB 42H;set bit 42H to (d)SETB 28H;set bit 28H to 1
(b)CLR 67H;clear bit (e)CLR 12 ;clear bit 12 (decimal)
(c)CLR 0FH;clear bit 0FH (f)SETB 05
Solution:
(a) RAM bit address of 42H belongs to D2 of RAM location 28H
(b) RAM bit address of 67H belongs to D7 of RAM location 2CH
(c) RAM bit address of 0FH belongs to D7 of RAM location 21H
(d) RAM bit address of 28H belongs to D0 of RAM location 25H
(e) RAM bit address of belongs to D4 of RAM location 21H
(f) RAM bit address of belongs to D5 of RAM location 20H

27. Example 8-9
The states of bits P1.2 and P1.3 of I/O port P1 must be saved before
they are changed. Write a program to save the status of P1.2 in bit
location 06 and the status of P1.3 in bit location 07.
Solution:
CLR ;clear bit address 06
CLR ;clear bit address 07
JNB P1.2,OVER ;If P1.2=0,jump
SETB ;
OVER:JNB P1.3,NEXT ;If P1.3=0,jump
SETB ;
NEXT:...

28. Section 8.2 Single-bit Operations with CY

29. Single-bit Operations
Instructions that are used for single-bit operations are given in Table 8-1.
These instructions can be used for any bit. 
Some instructions that allow single-bit operations, but only along with the carry flag (CY).
In 8051, there are several instructions by which the CY flag can be manipulated directly.
See Table 8-3. 

30. Table 8-1: Single-Bit Instructions
Function
SETB
bit
Set the bit (bit = 1)
CLR
Clear the bit (bit = 0)
CPL
Complement the bit (bit = NOT bit) JB
bit,target
Jump to target if bit = 1 (jump if bit)
JNB
Jump to target if bit = 0 (jump if no bit)
JBC
Jump to target if bit = 1, clear bit (jump if bit, then clear) 

31. Table 8-3: Carry Bit-Related Instructions
Function
SETB C
make CY = 1
CLR C
clear carry bit (CY = 0)
CPL C
complement carry bit
MOV b,C
copy carry status to bit location (CY = b)
MOV C,b
copy bit location status to carry (b = CY)
JNC target
jump to target if CY = 0
JC target
jump to target if CY = 1
ANL C,bit
AND CY with bit and save it on CY
ANL C,/bit
AND CY with inverted bit and save it on CY
ORL C,bit
OR CY with bit and save it on CY
ORL C,/bit
OR CY with inverted bit and save it on CY

32. Example 8-10
Write a program to save the status of bits P1.2 and P1.3 on RAM
bit locations 6 and 7, respectively.
Solution:
CY is used as a bit buffer.
MOV C,P1.2 ;save status of P1.2 on CY
MOV 06,C ;save in RAM bit location 06
MOV C,P1.3 ;save status of P1.3 on CY
MOV 07,C ;save in RAM bit location 07

33. Example 8-11 (1/2)
Assume that RAM bit location 12H holds the status of whether there has been a phone call or not.
If it is high, it means there has been a new call since it was checked the last time. Write a program to display “New Message” on an LCD if bit RAM 12H is high.
If it is low, the LCD should say “No New Messages”.
Solution:
Use CY to hold the status.
Use JNC to check CY flag.
We use “LCALL DISPLAY” to display message (see Chap. 12)

34. Example 8-11 (2/2) MOV C,12H ;copy bit location 12H to
JNC NO ;check to see if is high
MOV DPTR,#400H ;address of YES_MG
LCALL DISPLAY ;display
SJMP EXIT ;get out
NO: MOV DPTR,#420H
LCALL DISPLAY
EXIT:
;data to be displayed on LCD
ORG 400H
YES_MG: DB “New Messages”
ORG 420H
NO_MG: DB “No New Messages”
MOV C, 12H
Jump if CY＝0
Test
Not Jump if CY ≠0
Display NO_MG
Display YES_MG
SJMP EXIT
EXIT

35. Example 8-12
Assume that the bit P2.2 is used to control the outdoor light and bit
P2.5 to control the light inside a building. Show how to turn on the
outside light and turn off the inside one.
Solution:
STEB C ;CY=1
ORL C,P2.2 ;CY = P2.2 ORed with CY
MOV P2.2,C ;turn it “on”
CLR C ;CY=0
ANL C,P2.5 ;CY=P2.5 ANDed with CY
MOV P2.5,C ;turn it off

36. Section 8.3 Reading Input Pins vs. Port Latch

37. Reading Ports When reading ports, there are two possibilities:
Read the status of the input pin
Read the internal latch of the output pin
Readers must study and understand the material on the internal working of ports that is given in Appendix C2.
See Chapter 4.
Table 8-4 shows instructions to read external pins
Table 8-5 shows instructions to read latches

38. Table 8-4: Instructions For Reading an Input Port
Mnemonics
Examples
Description
MOV A,PX
MOV A,P2
Bring into A the data at P2 pins
JNB PX.Y,..
JNB P2.1,TARGET
Jump if pin P2.1 is low
JB PX.Y,..
JB P1.3,TARGET
Jump if pin P1.3 is high
MOV C,PX.Y
MOV C,P2.4
Copy status of pin P2.4 to CY

39. Table 8-5: Instructions Reading a Latch (Read-Modify-Write)
Mnemonics
Example
SETB P1.4
SETB PX.Y
CLR P1.3
CLR PX.Y
MOV P1.2,C
MOV PX.Y,C
DJNZ P1,TARGET
DJNZ PX, TARGET
INC P1
INC
CPL P1.2
CPL
JBC P1.1, TARGET
JBC PX.Y, TARGET
XRL P1,A
XRL
ORL P1,A
ORL
ANL P1,A
ANL
DEC P1
DEC

40. Read-modify-write Feature
The sequence of actions taken：
1. CPU reads the latch of the port
2. CPU perform the operation
3. Modifying the latch
4. Writing to the pin
Note that 8 pins of P1 work independently.
All read-modify-write instructions use the ports as the destination operand.
That is, these ports are configured as output ports.

41. You are able to
List the 8051 Assembly language instructions for bit manipulation
Code 8051 instructions for bit manipulation of ports
Explain which 8051 registers are bit-addressable
Describe which portions of the 8051 RAM are bit-addressable
Discuss bit manipulation of the carry flag
Describe the carry flag bit-related instructions of the 8051

42. Homework Chapter 8 Problems：7,16,24,25,40,41,44 Note:
Please write and compile the program of Problems 7,40,41,44.