memory Stacks in 8086 Microprocessor

Stacks

The stack is a block of memory that may be used for temporarily storing the contents of the registers inside the CPU. It is a top-down data structure whose elements are accessed using the stack pointer (SP) which gets decremented by two as we store a data word into the stack and gets incremented by two as we retrieve a data word from the stack back to the CPU register.

The process of storing the data in the stack is called ‘pushing into’ the stack and the

reverse process of transferring the data back from the stack to the CPU register is known as

‘popping off’ the stack. The stack is essentially Last-In-First-Out (LIFO) data segment. This

means that the data which is pushed into the stack last will be on top of stack and will be popped off the stack first.

The stack pointer is a 16-bit register that contains the offset address of the memory location in the stack segment. The stack segment, like any other segment, may have a memory block of a maximum of 64 Kbytes locations, and thus may overlap with any other segments. Stack Segment register (SS) contains the base address of the stack segment in the memory.

The Stack Segment register (SS) and Stack pointer register (SP) together address the stack-top as explained below:

If the stack top points to a memory location 52050H, it means that the location 52050H is already occupied with the previously pushed data. The next 16 bit push operation will decrement the stack pointer by two, so that it will point to the new stack-top 5204EH and the decremented contents of SP will be 204EH. This location will now be occupied by the recently pushed data.

Thus for a selected value of SS, the maximum value of SP=FFFFH and the segment can have maximum of 64K locations. If the SP starts with an initial value of FFFFH, it will be decremented by two whenever a 16-bit data is pushed onto the stack. After successive push operations, when the stack pointer contains 0000H, any attempt to further push the data to the stack will result in stack overflow.

After a procedure is called using the CALL instruction, the IP is incremented to the next instruction. Then the contents of IP, CS and flag register are pushed automatically to the stack. The control is then transferred to the specified address in the CALL instruction i.e. starting address of the procedure. Then the procedure is executed.