SECTION 3.2: CALL INSTRUCTIONS
Another control transfer instruction is the CALL instruction, which is used to call a subroutine. Subroutines are often used to perform tasks that need to be performed frequently. This makes a program more structured in addition to saving memory space. In the 8051 there are two instructions for call: LCALL (long call) and ACALL (absolute call). Deciding which one to use depends on the target address. Each instruction is explained next.
LCALL (long call)
In this 3-byte instruction, the first byte is the opcode and the second and third bytes are used for the address of the target subroutine. Therefore, LCALL can be used to call subroutines located anywhere within the 64K-byte address space of the 8051. To make sure that after execution of the called subroutine the 8051 knows where to come back to, the processor automatically saves on the stack the address of the instruction immediately below the LCALL. When a subroutine is called, 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. After finishing execution of the subroutine, the instruction RET (return) transfers control back to the caller. Every subroutine needs RET as the last instruction. See Example 3-8.
The following points should be noted for the program in Example 3-8.
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Notice the DELAY subroutine. Upon executing the first “LCALL DELAY”,
the address of the instruction right below it, “MOV A, #OAAH”, is pushed onto
the stack, and the 8051 starts to execute instructions at address 300H.
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In the DELAY subroutine, first the counter R5 is set to 255 (R5 = FFH); there
fore, the loop is repeated 256 times. When R5 becomes 0, control falls to the
RET instruction, which pops the address from the stack into the program
counter and resumes executing the instructions after the CALL.
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The amount of time delay in Example 3-8 depends on the frequency of the 8051. How to calculate the exact time will be explained in detail in Chapter 4. However you can increase the time delay by using a nested loop as shown below.
CALL instruction and the role of the stack
The stack and stack pointer were covered in the last chapter. To understand the importance of the stack in microcontrollers, we now examine the contents of the stack and stack pointer for Example 3-8. This is shown in Example 3-9.
Example 3-9
Use of PUSH and POP instructions in subroutines
Upon calling a subroutine, the stack keeps track of where the CPU should return after completing the subroutine. For this reason, we must be very careful in any manipulation of stack contents. The rule is that the number of PUSH and POP instructions must always match in any called subroutine. In other words, for every PUSH there must be a POP. See Example 3-10.
Calling subroutines
In Assembly language programming it is common to have one main program and many subroutines that are called from the main program. This allows you to make each subroutine into a separate module. Each module can be tested separately and then brought together with the main program. More importantly, in a large program the modules can be assigned to different programmers in order to shorten development time.
It needs to be emphasized that in using LCALL, the target address of the subroutine can be anywhere within the 64K-byte memory space of the 8051. This is not the case for the other call instruction, ACALL, which is explained next.
Figure 3-1. 8051 Assembly Main Program That Calls Subroutines
ACALL (absolute call)
ACALL is a 2-byte instruction in contrast to LCALL, which is 3 bytes. Since ACALL is a 2-byte instruction, the target address of the subroutine must be within 2K bytes because only 11 bits of the 2 bytes are used for the address. There is no difference between ACALL and LCALL in terms of saving the program counter on the stack or the function of the RET instruction. The only difference is that the target address for LCALL can be anywhere within the 64K-byte address space of the 8051 while the target address of ACALL must be within a 2K-byte range. In many variations of the 8051 marketed by different companies, on-chip ROM is as low as IK byte. In such cases, the use of ACALL instead of LCALL can save a number of bytes of program ROM space.
Example 3-11
A developer is using the Atmel AT89C1051 microcontroller chip for a product. This chip has only IK byte of on-chip flash ROM. Which instruction, LCALL or ACALL, is more useful in programming this chip?
Solution:
The ACALL instruction is more useful since it is a 2-byte instruction. It saves one byte each time the call instruction is used.
Of course, in addition to using compact instructions, we can program effe-ciently by having a detailed knowledge of all the instructions supported by a given microprocessor, and using them wisely. Look at Example 3-12.
Example 3-12
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