The flow of program proceeds in a sequential manner, from one instruction to the next instruction, unless a control transfer instruction is executed. The various types of control transfer instruction in assembly language include conditional or unconditional jumps and call instructions.
Loop and Jump Instructions
Looping in the 8051
Repeating a sequence of instructions a certain number of times is called a loop. An instruction DJNZ reg, label is used to perform a Loop operation. In this instruction, a register is decremented by 1; if it is not zero, then 8051 jumps to the target address referred to by the label.
The register is loaded with the counter for the number of repetitions prior to the start of the loop. In this instruction, both the registers decrement and the decision to jump are combined into a single instruction. The registers can be any of R0–R7. The counter can also be a RAM location.
Example
Multiply 25 by 10 using the technique of repeated addition.
Solution − Multiplication can be achieved by adding the multiplicand repeatedly, as many times as the multiplier. For example,
25 * 10 = 250(FAH)
25 + 25 + 25 + 25 + 25 + 25 + 25 + 25 + 25 + 25 = 250
MOV A,#0 ;A = 0,clean ACC MOV R2,#10 ; the multiplier is replaced in R2 Add A,#25 ;add the multiplicand to the ACC AGAIN:DJNZ R2, AGAIN:repeat until R2 = 0 (10 times) MOV R5 , A ;save A in R5 ;R5 (FAH)
Drawback in 8051 − Looping action with the instruction DJNZ Reg label is limited to 256 iterations only. If a conditional jump is not taken, then the instruction following the jump is executed.
Looping inside a Loop
When we use a loop inside another loop, it is called a nested loop. Two registers are used to hold the count when the maximum count is limited to 256. So we use this method to repeat the action more times than 256.
Example
Write a program to −
- Load the accumulator with the value 55H.
- Complement the ACC 700 times.
Solution − Since 700 is greater than 255 (the maximum capacity of any register), two registers are used to hold the count. The following code shows how to use two registers, R2 and R3, for the count.
MOV A,#55H ;A = 55H NEXT: MOV R3,#10 ;R3 the outer loop counter AGAIN:MOV R2,#70 ;R2 the inner loop counter CPL A ;complement
Other Conditional Jumps
The following table lists the conditional jumps used in 8051 −
| Instruction | Action |
|---|---|
| JZ | Jump if A = 0 |
| JNZ | Jump if A ≠ 0 |
| DJNZ | Decrement and Jump if register ≠ 0 |
| CJNE A, data | Jump if A ≠ data |
| CJNE reg, #data | Jump if byte ≠ data |
| JC | Jump if CY = 1 |
| JNC | Jump if CY ≠ 1 |
| JB | Jump if bit = 1 |
| JNB | Jump if bit = 0 |
| JBC | Jump if bit = 1 and clear bit |
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JZ (jump if A = 0) − In this instruction, the content of the accumulator is checked. If it is zero, then the 8051 jumps to the target address. JZ instruction can be used only for the accumulator, it does not apply to any other register.
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JNZ (jump if A is not equal to 0) − In this instruction, the content of the accumulator is checked to be non-zero. If it is not zero, then the 8051 jumps to the target address.
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JNC (Jump if no carry, jumps if CY = 0) − The Carry flag bit in the flag (or PSW) register is used to make the decision whether to jump or not “JNC label”. The CPU looks at the carry flag to see if it is raised (CY = 1). If it is not raised, then the CPU starts to fetch and execute instructions from the address of the label. If CY = 1, it will not jump but will execute the next instruction below JNC.
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JC (Jump if carry, jumps if CY = 1) − If CY = 1, it jumps to the target address.
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JB (jump if bit is high)
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JNB (jump if bit is low)
Note − It must be noted that all conditional jumps are short jumps, i.e., the address of the target must be within –128 to +127 bytes of the contents of the program counter.
Unconditional Jump Instructions
There are two unconditional jumps in 8051 −
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LJMP (long jump) − LJMP is 3-byte instruction in which the first byte represents opcode, and the second and third bytes represent the 16-bit address of the target location. The 2-byte target address is to allow a jump to any memory location from 0000 to FFFFH.
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SJMP (short jump) − It is a 2-byte instruction where the first byte is the opcode and the second byte is the relative address of the target location. The relative address ranges from 00H to FFH which is divided into forward and backward jumps; that is, within –128 to +127 bytes of memory relative to the address of the current PC (program counter). In case of forward jump, the target address can be within a space of 127 bytes from the current PC. In case of backward jump, the target address can be within –128 bytes from the current PC.
Calculating the Short Jump Address
All conditional jumps (JNC, JZ, and DJNZ) are short jumps because they are 2-byte instructions. In these instructions, the first byte represents opcode and the second byte represents the relative address. The target address is always relative to the value of the program counter. To calculate the target address, the second byte is added to the PC of the instruction immediately below the jump. Take a look at the program given below −
Line PC Op-code Mnemonic Operand 1 0000 ORG 0000 2 0000 7800 MOV R0,#003 3 0002 7455 MOV A,#55H0 4 0004 6003 JZ NEXT 5 0006 08 INC R0 6 0007 04 AGAIN: INC A 7 0008 04 INC A 8 0009 2477 NEXT: ADD A, #77h 9 000B 5005 JNC OVER 10 000D E4 CLR A 11 000E F8 MOV R0, A 12 000F F9 MOV R1, A 13 0010 FA MOV R2, A 14 0011 FB MOV R3, A 15 0012 2B OVER: ADD A, R3 16 0013 50F2 JNC AGAIN 17 0015 80FE HERE: SJMP HERE 18 0017 END
Backward Jump Target Address Calculation
In case of a forward jump, the displacement value is a positive number between 0 to 127 (00 to 7F in hex). However, for a backward jump, the displacement is a negative value of 0 to –128.
CALL Instructions
CALL is used to call a subroutine or method. Subroutines are used to perform operations or tasks that need to be performed frequently. This makes a program more structured and saves memory space. There are two instructions − LCALL and ACALL.
LCALL (Long Call)
LCALL is a 3-byte instruction where the first byte represents the opcode and the second and third bytes are used to provide the address of the target subroutine. LCALL can be used to call subroutines which are available within the 64K-byte address space of the 8051.
To make a successful return to the point after execution of the called subroutine, the CPU saves the address of the instruction immediately below the LCALL on the stack. Thus, when a subroutine is called, the control is transferred to that subroutine, and the processor saves the PC (program counter) on the stack and begins to fetch instructions from the new location. The instruction RET (return) transfers the control back to the caller after finishing execution of the subroutine. Every subroutine uses RET as the last instruction.
ACALL (Absolute Call)
ACALL is a 2-byte instruction, in contrast to LCALL which is 3 bytes. The target address of the subroutine must be within 2K bytes because only 11 bits of the 2 bytes are used for address. The difference between the ACALL and LCALL is that the target address for LCALL can be anywhere within the 64K-bytes address space of the 8051, while the target address of CALL is within a 2K-byte range.
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