In
IBM terminology, an Access Register (AR) is a
hardware register in
ESA/370 and later
processors. 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 st ...
s simultaneously. ARs were introduced with
ESA/370 in 1988, and supported by the
MVS/ESA operating system.
In
IBM System/360 architecture 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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