On the 11/20, this was accomplished by clipping the lead to the low-order bit of the stack register.

There was no dedicated stack register.

There was no stack register, but later Eclipse designs would utilize a dedicated hardware memory address for this function.

Now you have a copy of the 5th item of the array sitting in the stack register for you to use in your program.

A stack register is a computer central processor register whose purpose is to keep track of a call stack.

If an operand is "pushed" out of B, the stack register is incremented and the operand placed at the resulting address.

Furthermore, the contents in ST(0) can be exchanged with another stack register using an instruction called FXCH ST(x).

In the Opcode column, i denotes the number of the floating point stack register used as the second operand (the first is always the ST(o) register - the top of the stack).

It copies the contents of one of the floating point stack register, depending on the contents of EFLAGS integer flag register, to the ST(0) (top of stack) register.

The major differences between the original 2116 architecture and the BPC microprocessor are a completely redesigned I/O structure, the removal of multiple levels of indirect addressing, and the provision of a stack register for subroutine call and return.

Combined with a fixed length for machine opcodes this machine limitation can make the jump to ESP technique almost impossible to implement (with the one exception being when the program actually contains the unlikely code to explicitly jump to the stack register).