THE PROGRAM COUNTER AND ROM SPACE IN THE 8051

SECTION 2.4: THE PROGRAM COUNTER AND ROM SPACE IN THE 8051
In this section we examine the role of the program counter (PC) register in executing an 8051 program. We also discuss ROM memory space for various 8051 family members.
Program counter in the 8051
Another important register in the 8051 is the PC (program counter). The program counter points to the address of the next instruction to be executed. As the CPU fetches the opcode from the program ROM, the program counter is incremented to point to the next instruction. The program counter in the 8051 is 16 bits wide. This means that the 8051 can access program addresses 0000 to FFFFH, a total of 64K bytes of code. However, not all members of the 8051 have the entire 64K bytes of on-chip ROM installed, as we will see soon. Where does the 8051 wake up when it is powered? We will discuss this important topic next.
Where the 8051 wakes up when it is powered up
One question that we must ask about any microcontroller (or microprocessor) is: At what address does the CPU wake up upon applying power to it? Each microprocessor is different. In the case of the 8051 family (that is, all members regardless of the maker and variation), the microcontroller wakes up at memory address 0000 when it is powered up. By powering up we mean applying Vcc to the RESET pin as discussed in Chapter 4. In other words, when the 8051 is powered up,.the PC (program counter) has the value of 0000 in it. This means that it expects the first opcode to be stored at ROM address OOOOH. For this reason in the 8051 system, the first opcode must be burned into memory location OOOOH of program ROM since this is where it looks for the first instruction when it is booted. We achieve this by the ORG statement in the source program as shown earlier. Next, we discuss the step-by-step action of the program counter in fetching and executing a sample program.
Placing code in program ROM
To get a better understanding of the role of the program counter in fetching and executing a program, we examine the action of the program counter as each instruction is fetched and executed. First, we examine once more the list file

of the sample program and how the code is placed in the ROM of an 8051 chip. As we can see, the opcode and operand for each instruction are listed on the left side of the list file.









Program21:ListFile


ROM Address
After the prografn is burned into ROM of an 8051 family member such as 8751 or AT8951 or DS5000, the opcode and operand are placed in ROM memory locations starting at 0000 as shown in the list below.



The list shows that address 0000 contains 7D, which is the opcode for moving a value into register R5, and address 0001 contains the operand (in this case 25H) to be moved to R5. Therefore, the instruction “MOV R5,#25H” has a machine code of “7D25″, where 7D is the opcode and 25 is the operand. Similarly, the machine code “7F34″ is located in memory locations 0002 and 0003 and represents the opcode and the operand for the instruction “MOV R7,#34H”. In the same way, machine code “7400″ is located in memory locations 0004 and 0005 and represents the opcode and the operand for the instruction “MOV A, #0″. The memory location 0006 has the opcode of 2D, which is the opcode for the

instruction “ADD A, R5″ and memory location 0007 has the content 2F, which is the opcode for the “ADD A, R7″ instruction. The opcode for the instruction “ADD A, #12H” is located at address 0008 and the operand 12H at address 0009. The memory location OOOA has the opcode for the SJMP instruction and its target address is located in location OOOB. The reason the target address is FE is explained in the next chapter.
Executing a program byte by byte
Assuming that the above program is burned into the ROM of an 8051 chip (or 8751, AT8951, or DS5000), the following is a step-by-step description of the action of the 8051 upon applying power to it.
  1. When the 8051 is powered up, the PC (program counter) has 0000 and starts
    to fetch the first opcode from location 0000 of the program ROM. In the case
    of the above program the first opcode is 7D, which is the code for moving an
    operand to R5. Upon executing the opcode, the CPU fetches the value 25 and
    places it in R5. Now one instruction is finished. Then the program counter is
    incremented to point to 0002 (PC = 0002), which contains opcode 7F, the
    opcode for the instruction “MOV R7 , . .”.
  2. Upon executing the opcode 7F, the value 34H is moved into R7. Then the pro
    gram counter is incremented to 0004.
  3. ROM location 0004 has the opcode for the instruction “MOV A, #0″. This
    instruction is executed and now PC = 0006. Notice that all the above instruc
    tions are 2-byte instructions; that is, each one takes two memory locations.
  4. Now PC = 0006 points to the next instruction, which is “ADD A, R5″. This is
    a 1-byte instruction. After the execution of this instruction, PC = 0007.
  5. The location 0007 has the opcode 2F, which belongs to the instruction “ADD
    A,R7″. This also is a 1-byte instruction. Upon execution of this instruction,
    PC is incremented to 0008. This process goes on until all the instructions are
    fetched and executed. The fact that the program counter points at the next
    instruction to be executed explains why some microprocessors (notably the
    x86) call the program counter the instruction pointer.
ROM memory map in the 8051 family

As we saw in the last chapter, some family members have only 4K bytes of on-chip ROM (e.g., 8751, AT8951) and some, such as the AT89C52, have 8K bytes of ROM. Dallas Semiconductor’s DS5000-32 has 32K bytes of on-chip ROM. Dallas Semiconductor also has an 8051 with 64K bytes of on-chip ROM. The point to remember is that no member of the 8051 family can access more than 64K bytes of opcode since the program counter in the 8051 is a 16-bit register (0000 to FFFF address range). It must be noted that while the first location of program ROM inside the 8051 has the address of 0000, the last location can be different depending on the size of the ROM on the chip. Among the 8051 family members, the 8751 and AT8951 have 4K bytes of on-chip ROM. This 4K bytes of ROM memory has memory addresses of 0000 to OFFFH. Therefore, the first location of on-chip ROM of this 8051 has an address of 0000 and the last location has the address of OFFFH. Look at Example 2-1 to see how this is computed.


Example 2-1
Find the ROM memory address of each of the following 8051 chips.
(a) AT89C51 with 4KB (b) DS89C420 with 16KB (c) DS5000-32 with 32KB
Solution:


Figure 2-3. 8051 On-Chip ROM Address Range
(a) With 4K bytes of on-chip ROM memory space, we have 4096 bytes (4 x 1024 = 4096). This maps to address locations of 0000 to OFFFH. Notice that 0 is always the first location, (b) With 16K bytes of on-chip ROM memory space, we have 16,384 bytes (16 x 1024 = 16,384), which gives 0000 – 3FFFH. (c) With 32K bytes we have 32,768 bytes (32 x 1024 = 32,768). Converting 32,768 to hex, we get 8000H; therefore, the memory space is 0000 to 7FFFH.

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