In
IBM terminology, an Access Register (AR) is a
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 ...
in
ESA/370
IBM Enterprise Systems Architecture is an instruction set architecture introduced by IBM as ESA/370 in 1988. It is based on the IBM System/370-XA architecture.
It extended the dual-address-space mechanism introduced in later IBM System/370 mo ...
and later
processors
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, ...
. Access registers work in conjunction with the
general purpose register
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. ...
s, giving a program transparent access to up to sixteen 2 GB
address spaces simultaneously. ARs were introduced with
ESA/370
IBM Enterprise Systems Architecture is an instruction set architecture introduced by IBM as ESA/370 in 1988. It is based on the IBM System/370-XA architecture.
It extended the dual-address-space mechanism introduced in later IBM System/370 mo ...
in 1988, and supported by the
MVS/ESA
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated ...
operating system.
In
IBM System/360 architecture
The IBM System/360 architecture is the model independent architecture for the entire S/360 line of mainframe computers, including but not limited to the instruction set architecture. The elements of the architecture are documented in the '' ...
all instructions address memory by specifying a 12-bit offset (4096 bytes) from a value in a "base register" with optional indexing. Originally addresses occupied the low-order 24 bits of a base register, allowing a program to access up to 16 MB.
System/370-XA extended the architecture to allow 31-bit addressing and address spaces of up to 2 GB.
Enterprise Systems Architecture/370 further expanded addressing capabilities with access registers. Sixteen 32-bit access registers "shadow" the sixteen general-purpose registers. In a processor mode called ''access-register mode'' the access register corresponding to the specified base register designates the operand address space to be accessed.
The contents of an access register is called an "Access-list entry token" (ALET), which contains an index into a system table identifying the address space.
See also
*
Address register
In a computer, the memory address register (MAR) is the CPU register that either stores the memory address from which data will be fetched to the CPU registers, or the address to which data will be sent and stored via system bus.
In other words, ...
References
IBM mainframe operating systems
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