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

call stack In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or m ...

. On an accumulator-based architecture machine, this may be a dedicated register. On a machine with mulitple

general-purpose registers A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. ...

, it may be a register that is reserved by convention, such as on the IBM System/360 through

z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture ...

architecture and RISC architectures, or it may be a register that procedure call and return instructions are hardwired to use, such as on the PDP-11,

VAX VAX (an acronym for Virtual Address eXtension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The V ...

, and

Intel x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

architectures. Some designs such as the

Data General Eclipse The Data General Eclipse line of computers by Data General were 16-bit minicomputers released in early 1974 and sold until 1988. The Eclipse was based on many of the same concepts as the Data General Nova, but included support for virtual memory ...

had no dedicated register, but used a reserved hardware memory address for this function. Machines before the late 1960s—such as the

PDP-8 The PDP-8 is a 12-bit minicomputer that was produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's lifetime. Its basic design follows the pioneer ...

and

HP 2100 The HP 2100 is a series of 16-bit minicomputers that were produced by Hewlett-Packard (HP) from the mid-1960s to early 1990s. Tens of thousands of machines in the series were sold over its twenty-five year lifetime, making HP the fourth largest mi ...

—did not have compilers which supported

recursion Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathemati ...

. Their subroutine instructions typically would save the current location in the jump address, and then set the program counter to the ''next'' address. While this is simpler than maintaining a stack, since there is only one return location per subroutine code section, there cannot be recursion without considerable effort on the part of the programmer. A stack machine has 2 or more stack registers — one of them keeps track of a

, the other(s) keep track of other stack(s).

Stack registers in x86

8086 The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...

, the main stack register is called stack pointer - SP. The stack segment register (SS) is usually used to store information about the

memory segment Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifie ...

that stores the

of currently executed program. SP points to current stack top. By default, the stack grows downward in memory, so newer values are placed at lower memory addresses. To push a value to the stack, the PUSH instruction is used. To pop a value from the stack, the POP instruction is used. Example: Assuming that SS = 1000h and SP = 0xF820. This means that current stack top is the physical address 0x1F820 (this is due to memory segmentation in 8086). The next two machine instructions of the program are: PUSH AX PUSH BX * These first instruction shall push the value stored in AX (16-bit register) to the stack. This is done by subtracting a value of 2 (2 bytes) from SP. * The new value of SP becomes 0xF81E. The CPU then copies the value of AX to the memory word whose physical address is 0x1F81E. * When "PUSH BX" is executed, SP is set to 0xF81C and BX is copied to 0x1F81C. This illustrates how PUSH works. Usually, the running program pushes registers to the stack to make use of the registers for other purposes, like to call a routine that may change the current values of registers. To restore the values stored at the stack, the program shall contain machine instructions like this: POP BX POP AX * POP BX copies the word at 0x1F81C (which is the old value of BX) to BX, then increases SP by 2. SP now is 0xF81E. * POP AX copies the word at 0x1F81E to AX, then sets SP to 0xF820.

Stack engine

Simpler processors store the stack pointer in a regular

hardware register In digital electronics, especially computing, hardware registers are circuits typically composed of flip flops, often with many characteristics similar to memory, such as: * The ability to read or write multiple bits at a time, and * Using an ...

and use the arithmetic logic unit (ALU) to manipulate its value. Typically push and pop are translated into multiple

micro-op In computer central processing units, micro-operations (also known as micro-ops or μops, historically also as micro-actions) are detailed low-level instructions used in some designs to implement complex machine instructions (sometimes termed ...

s, to separately add/subtract the stack pointer, and perform the load/store in memory. Newer processors contain a dedicated stack engine to optimize stack operations.

Pentium M The Pentium M is a family of mobile 32-bit single-core x86 microprocessors (with the modified Intel P6 microarchitecture) introduced in March 2003 and forming a part of the Intel Carmel notebook platform under the then new Centrino brand. The ...

was the first x86 processor to introduce a stack engine. In its implementation, the stack pointer is split among two registers: ESP_O, which is a 32-bit register, and ESP_d, an 8-bit delta value that is updated directly by stack operations. PUSH, POP, CALL and RET opcodes operate directly with the ESP_d register. If ESP_d is near overflow or the ESP register is referenced from other instructions (when ESP_d ≠ 0), a synchronisation micro-op is inserted that updates the ESP_O using the ALU and resets ESP_d to 0. This design has remained largely unmodified in later Intel processors, although ESP_O has been expanded to 64 bits. A stack engine similar to Intel's was also adopted in the

AMD K8 The AMD K8 Hammer, also code-named SledgeHammer, is a computer processor microarchitecture designed by AMD as the successor to the AMD K7 Athlon microarchitecture. The K8 was the first implementation of the AMD64 64-bit extension to the x86 ins ...

microarchitecture. In Bulldozer, the need for synchronization micro-ops was removed, but the internal design of the stack engine is not known.

Notes

References

{{CPU technologies Control flow Central processing unit Digital registers