The computers which use Stack-based CPU Organization are based on a data structure called a stack. The stack is a list of data words. It uses the Last In First Out (LIFO) access method which is the most popular access method in most of the CPU. A register is used to store the address of the topmost element of the stack which is known as Stack pointer (SP). In this organization, ALU operations are performed on stack data. It means both the operands are always required on the stack. After manipulation, the result is placed in the stack.
The main two operations that are performed on the operators of the stack are Push and Pop. These two operations are performed from one end only.
1. Push –
This operation results in inserting one operand at the top of the stack and it increases the stack pointer register. The format of the PUSH instruction is:
It inserts the data word at a specified address to the top of the stack. It can be implemented as:
// Increment SP by 1
SP <-- SP + 1
//store the content of specified memory address
//into SP; i.e, at top of stack
SP <-- (memory address)
2. Pop –
This operation results in deleting one operand from the top of the stack and decreasing the stack pointer register. The format of the POP instruction is:
It deletes the data word at the top of the stack to the specified address. It can be implemented as:
//transfer the content of SP (i.e, at top most data)
//into specified memory location
(memory address) <-- SP
//Decrement SP by 1
SP <-- SP - 1
Operation type instruction does not need the address field in this CPU organization. This is because the operation is performed on the two operands that are on the top of the stack. For example:
This instruction contains the opcode only with no address field. It pops the two top data from the stack, subtracting the data, and pushing the result into the stack at the top.
PDP-11, Intel’s 8085, and HP 3000 are some examples of stack-organized computers.
The advantages of Stack-based CPU organization –
- Efficient computation of complex arithmetic expressions.
- Execution of instructions is fast because operand data are stored in consecutive memory locations.
- The length of instruction is short as they do not have an address field.
The disadvantages of Stack-based CPU organization –
- The size of the program increases.
Note: Stack-based CPU organization uses zero address instruction.