Editing Stack machine (section)

===Hybrid machines===
{{distinguish|text=[[hybrid computer]]s that combine both digital and analogue features}}

Pure stack machines are quite inefficient for procedures which access multiple fields from the same object. The stack machine code must reload the object pointer for each pointer+offset calculation. A common fix for this is to add some register-machine features to the stack machine: a visible register file dedicated to holding addresses, and register-style instructions for doing loads and simple address calculations. It is uncommon to have the registers be fully general purpose, because then there is no strong reason to have an expression stack and postfix instructions.

Another common hybrid is to start with a register machine architecture, and add another memory address mode which emulates the push or pop operations of stack machines: 'memaddress = reg; reg += instr.displ'. This was first used in [[Digital Equipment Corporation|DEC]]'s [[PDP-11]] minicomputer.<ref name="Duncan_1977"/> This feature was carried forward in [[VAX]] computers and in [[Motorola 6809]] and [[Motorola 68000|M68000]] microprocessors. This allowed the use of simpler stack methods in early compilers. It also efficiently supported virtual machines using stack interpreters or [[threaded code]]. However, this feature did not help the register machine's own code to become as compact as pure stack machine code. Also, the execution speed was less than when compiling well to the register architecture. It is faster to change the top-of-stack pointer only occasionally (once per call or return) rather than constantly stepping it up and down throughout each program statement, and it is even faster to avoid memory references entirely.

More recently, so-called second-generation stack machines have adopted a dedicated collection of registers to serve as address registers, off-loading the task of memory addressing from the data stack. For example, MuP21 relies on a register called "A", while the more recent GreenArrays processors relies on two registers: A and B.<ref name="Colorforth_F18A"/>

The Intel x86 family of microprocessors have a register-style (accumulator) instruction set for most operations, but use stack instructions for its [[x87]], [[Intel 8087]] floating point arithmetic, dating back to the iAPX87 (8087) coprocessor for the 8086 and 8088. That is, there are no programmer-accessible floating point registers, but only an 80-bit wide, 8-level deep stack. The x87 relies heavily on the x86 CPU for assistance in performing its operations.