Embedded Systems - Terms

Program Counter

The Program Counter is a 16- or 32-bit register which contains the address of the next instruction to be executed. The PC automatically increments to the next sequential memory location every time an instruction is fetched. Branch, jump, and interrupt operations load the Program Counter with an address other than the next sequential location.

Activating a power-on reset will cause all values in the register to be lost. It means the value of the PC (program counter) is 0 upon reset, forcing the CPU to fetch the first opcode from the ROM memory location 0000. It means we must place the first byte of upcode in ROM location 0000 because that is where the CPU expects to find the first instruction

Reset Vector

The significance of the reset vector is that it points the processor to the memory address which contains the firmware's first instruction. Without the Reset Vector, the processor would not know where to begin execution. Upon reset, the processor loads the Program Counter (PC) with the reset vector value from a predefined memory location. On CPU08 architecture, this is at location $FFFE:$FFFF.

When the reset vector is not necessary, developers normally take it for granted and don’t program into the final image. As a result, the processor doesn't start up on the final product. It is a common mistake that takes place during the debug phase.

Stack Pointer

Stack is implemented in RAM and a CPU register is used to access it called SP (Stack Pointer) register. SP register is an 8-bit register and can address memory addresses of range 00h to FFh. Initially, the SP register contains value 07 to point to location 08 as the first location being used for the stack by the 8051.

When the content of a CPU register is stored in a stack, it is called a PUSH operation. When the content of a stack is stored in a CPU register, it is called a POP operation. In other words, a register is pushed onto the stack to save it and popped off the stack to retrieve it.

Infinite Loop

An infinite loop or an endless loop can be identified as a sequence of instructions in a computer program that executes endlessly in a loop, because of the following reasons −

  • loop with no terminating condition.
  • loop with a terminating condition that can never be met.
  • loop with a terminating condition that causes the loop to start over.

Such infinite loops normally caused older operating systems to become unresponsive, as an infinite loop consumes all the available processor time. I/O operations waiting for user inputs are also called "infinite loops". One possible cause of a computer "freezing" is an infinite loop; other causes include deadlock and access violations.

Embedded systems, unlike a PC, never "exit" an application. They idle through an Infinite Loop waiting for an event to take place in the form of an interrupt, or a pre-scheduled task. In order to save power, some processors enter special sleep or wait modes instead of idling through an Infinite Loop, but they will come out of this mode upon either a timer or an External Interrupt.


Interrupts are mostly hardware mechanisms that instruct the program that an event has occurred. They may occur at any time, and are therefore asynchronous to the program flow. They require special handling by the processor, and are ultimately handled by a corresponding Interrupt Service Routine (ISR). Interrupts need to be handled quickly. If you take too much time servicing an interrupt, then you may miss another interrupt.

Little Endian Vs Big Endian

Although numbers are always displayed in the same way, they are not stored in the same way in memory. Big-Endian machines store the most significant byte of data in the lowest memory address. A Big-Endian machine stores 0x12345678 as −

ADD+0: 0x12 
ADD+1: 0x34 
ADD+2: 0x56 
ADD+3: 0x78

Little-Endian machines, on the other hand, store the least significant byte of data in the lowest memory address. A Little-Endian machine stores 0x12345678 as −

ADD+0: 0x78 
ADD+1: 0x56 
ADD+2: 0x34 
ADD+3: 0x12