Repeating a sequence of instructions a certain number of times is called a loop

Repeating a sequence of instructions a certain number of times is called a loop

• Repeating a sequence of instructions a certain number of times is called a loop

• Loop action is performed by
• DJNZ reg, Label
• The register is decremented
• If it is not zero, it jumps to the target address referred to by the label
• Prior to the start of loop the register is loaded with the counter for the number of repetitions
• Counter can be R0 – R7 or RAM location
• A loop can be repeated a maximum of 256 times

If we want to repeat an action more times than 256, we use a loop inside a loop, which is called nested loop

• If we want to repeat an action more times than 256, we use a loop inside a loop, which is called nested loop

• We use multiple registers to hold the count

Jump only if a certain condition is met

• Jump only if a certain condition is met

• JZ (jump if A = 0)
• The content of register A is checked. If it is zero, it jumps to the target address.

JNZ (jump if A ≠ 0)

• JNZ (jump if A ≠ 0)
• The content of register A is checked. If it is not zero, it jumps to the target address.

JNC label ;jump if no carry, CY=0

• JNC label ;jump if no carry, CY=0
• If CY = 0, the CPU starts to fetch and execute instruction from the address of the label
• If CY = 1, it will not jump but will execute the next instruction below JNC

• All conditional jumps are short jumps

• The address of the target must within -128 to +127 bytes of the contents of PC

The unconditional jump is a jump in which control is transferred unconditionally to the target location

• The unconditional jump is a jump in which control is transferred unconditionally to the target location

• LJMP (long jump)
• 3-byte instruction
• First byte is the opcode
• Second and third bytes represent the 16-bit target address
• Any memory location from 0000 to FFFFH

2-byte instruction

• 2-byte instruction
• First byte is the opcode
• Second byte is the relative target address 00 to FFH
• Forward +127 and backward -128 bytes from the current PC

• To calculate the target address of a short jump (SJMP, JNC, JZ, DJNZ, etc.)

• The second byte is added to the PC of the instruction immediately below the jump
• If the target address is out of range, the assembler will generate an error

Call instruction is used to call subroutine

• Call instruction is used to call subroutine

• Subroutines are often used to perform tasks that need to be performed frequently
• This makes a program more structured inaddition to saving memory space
• LCALL (long call)
• 3-byte instruction
• First byte is the opcode
• Second and third bytes are used for address of target subroutine
• Subroutine is located anywhere within 64K byte address space

ACALL (absolute call)

• ACALL (absolute call)
• 2-byte instruction
• 11 bits are used for address within 2K-byte range

• When a subroutine is called, control is transferred to that subroutine

• The processor saves on the stack the the address of the instruction immediately below the CALL
• It also begins to fetch instructions form the new location

After finishing execution of the subroutine

• After finishing execution of the subroutine

• The instruction RET transfers control back to the caller
• Every subroutine needs RET as the last instruction
• RET pops the address from the stack into the PC and resumes executing the instructions after the CALL

The only difference between ACALL and LCALL is

• The only difference between ACALL and LCALL is

• The target address for LCALL can be anywhere within the 64K byte address
• The target address of ACALL must be within a 2K-byte range

• The use of ACALL instead of LCALL can save a number of bytes of program ROM space

CPU executing an instruction takes a certain number of clock cycles

• CPU executing an instruction takes a certain number of clock cycles

• These are referred as to as machine cycles

• The length of machine cycle depends on the frequency of the crystal oscillator connected to 8051

• In original 8051, one machine cycle lasts 12 oscillator periods

Two factors can affect the accuracy of the delay:

• Two factors can affect the accuracy of the delay:

• Crystal frequency
• The duration of the clock period of the machine cycle is a function of this crystal frequency
• 8051 design
• The original machine cycle duration was set at 12 clocks
• Advances in both IC technology and CPU design in recent years have made the 1-clock machine cycle a common feature