Index register

Index register display on anIBM 7094mainframe from the early 1960s

Anindex registerin a computer'sCPUis aprocessor register(or an assigned memory location)used for pointing tooperandaddresses during the run of a program. It is useful for stepping throughstringsandarrays.It can also be used for holding loop iterations and counters. In somearchitecturesit is used for read/writing blocks of memory. Depending on the architecture it may be a dedicated index register or a general-purpose register.Someinstruction setsallow more than one index register to be used; in that case additional instruction fields may specify which index registers to use.

Generally, the contents of an index register is added to (in some cases subtracted from) animmediateaddress (that can be part of the instruction itself or held in another register) to form the "effective" address of the actual data (operand). Special instructions are typically provided to test the index register and, if the test fails, increments the index register by an immediate constant and branches, typically to the start of the loop. While normally processors that allow an instruction to specify multiple index registers add the contents together, IBM had a line of computers in which the contents were or'd together.

Index registers has proved useful for doingvector/arrayoperations and in commercial data processing for navigating from field to field within records. In both uses index registers substantially reduced the amount of memory used and increased execution speed.

History

In early computers without any form ofindirect addressing,array operations had to be performed bymodifyingthe instruction address, which required several additional program steps and used up more computer memory,a scarce resource in computer installations of the early era (as well as in early microcomputers two decades later).

Index registers, commonly known as B-lines in early British computers, as B-registers on some machines and a X-registerson others, were first used in theBritishManchester Mark 1computer, in 1949. In general, index registers became a standard part of computers during the technology'ssecond generation,roughly 1954–1966. Mostmachines in theIBM 700/7000mainframeseries had them, starting with theIBM 704in 1954, though they were optional on some smaller machines such as theIBM 650andIBM 1401.

Early "small machines" with index registers include theAN/USQ-17,around 1960, and the9 seriesofreal-time computersfromScientific Data Systems,from the early 1960s.

The 1962UNIVAC 1107has 15 X-registers, four of which were also A-registers.

The 1964GE-635has 8 dedicated X-registers; however, it also allows indexing by the instruction counter or by either half of the A or Q register.

TheDigital Equipment Corporation(DEC)PDP-6,introduced in 1964, and theIBM System/360,announced in 1964, do not include dedicated index registers; instead, they havegeneral-purpose registers(called "accumulators" in the PDP-6) that can contain either numerical values or addresses. The memory address of an operand is, in the PDP-6, the sum of the contents of a general-purpose register and an 18-bit offset and, on the System/360, the sum of the contents of two general-purpose registers and a 12-bit offset.The compatiblePDP-10line of successors to the PDP-6, and theIBM System/370and later compatible successors to the System/360, including the currentz/Architecture,work in the same fashion.

The 1969Data General Novaand successorEclipse,and 1970 DECPDP-11,minicomputersalso provided general-purpose registers (called "accumulators" in the Nova and Eclipse), rather than separate accumulators and index registers, as did theirEclipse MVandVAX32-bitsuperminicomputersuccessors. In the PDP-11 and VAX, any register could be used when calculating the memory address of an operand; in the Nova, Eclipse, and Eclipse MV, only registers 2 and 3 could be used.

The 1971CDC STAR-100has a register file of 256 64-bit registers, 9 of which are reserved. Unlike most computers, the STAR-100 instructions only have register fields and operand fields, so the registers serve more as pointer registers than as traditional index registers.

While the 1972Intel 8008allowed indirect addressing via register pairs, the firstmicroprocessorwith a true index register appears to have been the 1974Motorola 6800.

In 1975, the 8-bitMOS Technology 6502processor had two index registers 'X' and 'Y'.

In 1978, theIntel 8086,the firstx86processor, had eight 16-bit registers, referred to as "general-purpose", all of which can be used as integer data registers in most operations; four of them, 'SI' (source index), 'DI' (destination index), 'BX' (base), and 'BP' (base pointer), can also be used when computing the memory address of an operand, which is the sum of one of those registers and a displacement, or the sum of one of 'BX' or 'BP ", one of 'SI' or 'DI', and a displacement.The 1979Intel 8088,and the 16-bitIntel 80186,Intel 80188,andIntel 80286successors work the same. In 1985, thei386,a 32-bit successor to those processors, introducing theIA-3232-bit version of the x86 architecture, extended the eight 16-bit registers to 32 bits, with "E" added to the beginning of the register name; in IA-32, the memory address of an operand is the sum of one of those eight registers, one of seven of those registers (the stack pointer is not allowed as the second register here) multiplied by 1, 2, 4, or 8, and a displacement.TheAdvanced Micro DevicesOpteron,the first model of which was released in 2003, introducedx86-64,the 64-bit version of the x86 instruction set; in x86-64, the general-purpose registers were extended to 64 bits, and eight additional general-purpose registers were added; the memory address of an operand is the sum of two of those 16 registers and a displacement.

Thereduced instruction set computing(RISC) instruction sets introduced in the 1980s and 1990s all provide general-purpose registers that can contain either numerical values or address values. In most of those instruction sets, there are 32 general-purpose registers (in some of those instruction sets, the value of one of those registers is hardwired to zero) could be used to calculate the operand address; they did not have dedicated index registers. In the 32-bit version of theARM architecture,first developed in 1985, there are 16 registers designated as "general-purpose registers", but only 13 of them can be used for all purposes, with register R15 containing theprogram counter.The memory address of a load or store instruction is the sum of any of the 16 registers and either a displacement or another of the registers with the exception of R15 (possibly shifted left for scaling).In the 64-bit version of the ARM architecture, there are 31 64-bit general-purpose registers plus a stack pointer and a zero register; the memory address of a load or store instruction is the sum of any of the 31 registers and either a displacement or another of the registers.

Examples

Here is a simple example of index register use in assembly language pseudo-code that sums a 100 entry array of 4-byte words:

Clear_accumulator
Load_index 400,index2 //load 4*array size into index register 2 (index2)
loop_start: Add_word_to_accumulator array_start,index2 //Add to AC the word at the address (array_start + index2)
Branch_and_decrement_if_index_not_zero loop_start,4,index2 //loop decrementing by 4 until index register is zero

See also

• For loop