1. Lecture Set 4
Programming the 8051

2. Addressing Modes Five addressing modes are available:
Immediate
Register
Direct
Indirect
Indexed
There are three more modes:
Relative
Absolute
Long
These are used with calls, branches and jumps and are handled automatically by the assembler.

3. Immediate Addressing
The data is directly specified in the instruction.
Useful for getting constants into registers.
Immediate data must be preceded with a “#” sign.
MOV R0, #0F0H ; Load R0 with the value F0H
The immediate value is a maximum of 8-bits.
One exception, when dealing with the DPTR register it can be 16-bits.
MOV DPTR, #2000H ; Load the value 2000H into the DPTR register

4. Register Addressing Mode
Direct access to eight registers – R0 through R7.
MOV A, R0
MOV R1, A
ADD A, R1
Not all combinations are valid.
MOV R2, R1 ; Invalid
There are 4 banks of registers accessible through register addressing.
Only one bank can be accessed at a time controllable through bit RS0 and RS1 of the PSW.
MOV PSW, # B
Set RS0:RS1 to 11, therefore, accessing register bank 3.

5. Direct Addressing
Direct addressing can access any on-chip hardware register.
All on-chip memory locations and registers have 8-bit addresses.
Can use the 8-bit address in the instruction.
MOV A, 4H ; Amem[04H]
Or can use the register name.
MOV A, R4
Don’t get confused with Immediate mode.
No “#” sign.

6. Indirect Addressing
R0 and R1 may be used as pointer registers where their contents indicate an address in internal RAM where the data is to be read or written.
MOV R1, #40H ; Make R1 point to location 40
MOV ; Move the contents of 40H to A
R1 ; Move contents of R1 into the memory location pointed to by R0.

7. Indirect Addressing Can also be used for accessing external memory:
Can use R0 and R1 to point to external memory locations 00H to FFH.
MOVX ; Move contents of external memory location whose address is in R1 into A
Can also use DPTR to point to all 64k of external memory.
MOVX

8. Indexed Addressing
Use a register for storing a pointer to memory and another register for storing an offset.
The effective address is the sum of the two:
EA = Pointer + Offset
MOVC ; Move byte from memory located at DPTR+A to A.

9. Example Program to implement X2 as a lookup table.
Supports 1 through 9 only.
ORG 0 ; assembler directive
mov DPTR, #LUT ; 300H is the LUT address
mov A, #0FFH
mov P1, A ; program the port P1 to input data
Again: mov A, P1 ; read x
movc ; get x2 from LUT
mov P2, A ; output x2 to P2
sjmp again ; for (1) loop
ORG 300H ;Look-up Table starts at 0x0300
LUT: DB 0, 1, 4, 9, 16, 25, 36, 49, 64, 81

10. Program Control Instructions
Unconditional Branch
ajmp addr11 ; absolute jump
ljmp addr16 ; long jump
sjmp rel ; short jump to relative address
; jump indirect
Conditional branch
jz, jnz rel ; short conditional jump to rel. addr
djnz rel ; decrement and jump if not zero
cjne rel ; compare and jump if not equal
Subroutine Call
acall addr11 ; absolute subroutine call
lcall addr16 ; long subroutine call
ret ; return from subroutine call
reti ; return from ISV

11. Target Address Target address can be,
absolute: A complete physical address
addr16: 16 bit address, anywhere in the 64k
addr11: 11 bit address, anywhere within 2k page.
rel: relative (forward or backward) -128 bytes to +127 bytes from the current code location
Target address calculation for relative jumps
PC of next instruction + rel address
For jump backwards, drop the carry
PC = 15H, SJMP 0FEH
Address is 15H + FEH = 13H
Basically jump to next instruction minus two (current instruction)

12. Conditional Jumps jz, jnz : Conditional on A==0
Checks to see if A is zero
jz jumps if A is zero and jnz jumps is A not zero
No arithmetic op need be performed (unlike 8086/8085)
djnz : dec a byte and jump if not equal to zero
djnz Rn, rel
djnz direct, rel
jnc : Conditional on carry CY flag
jc rel
jnc rel
cjne : compare and jump if not equal
cjne A, direct, rel
cjne Rn, #data, rel
#data, rel

13. Loop using djnz Add 3 to A ten times Loop within loop using djnz
mov A, #0 ; clear A
mov R2, #10 ; R2  10, can also say 0AH
AGAIN: add A, #03 ; add 3 to A
djnz R2, AGAIN ; repeat until R2==0
mov R5, A ; save the result in R5
Loop within loop using djnz
mov R3, #100
loop1: mov R2, #10 ; trying for 1000 loop iterations
loop2: nop ; no operation
djnz R2, loop2 ; repeat loop2 until R2==0
djnz R3, loop1 ; repeat loop1 until R3==0

14. Unconditional Jumps LJMP addr16 SJMP rel Long jump. 3 byte instruction
Jump to a 2byte target address
3 byte instruction
SJMP rel
Jump to a relative address from PC+127 to PC-128
Jump to PC (00H – 7FH)
Jump to PC – 128 (80H – FFH)

15. Call Instructions Subroutines: LCALL addr16 ACALL addr11 RET
Reusable code snippets
LCALL addr16
Long call.
3 byte instruction.
Call any subroutine in entire 64k code space
PC is stored on the stack
ACALL addr11
2 byte instruction
Call any subroutine within 2k of code space
Other than this, same behavior as LCALL
Saves code ROM for devices with less than 64K ROM
RET
Return from a subroutine call, Pops PC from stack