In
IBM
International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...
terminology, an Access Register (AR) is a
hardware register in
ESA/370 and later mainframe
instruction set architectures
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, ...
. Access registers work in conjunction with the
general purpose registers, giving a program transparent access to up to sixteen 2 GB
address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.
For software programs to save and retrieve ...
s simultaneously. ARs were introduced with ESA/370 in 1988, and supported by the
MVS/ESA 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 ''IB ...
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
References
IBM mainframe operating systems
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