# Access register

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In [IBM](/source/IBM) terminology, an **Access Register** (AR) is a [hardware register](/source/Hardware_register) in [ESA/370](/source/ESA/370) and later mainframe [instruction set architectures](/source/Instruction_set_architectures). Access registers work in conjunction with the [general purpose registers](/source/General_purpose_register), giving a program transparent access to up to sixteen 2 GB [address spaces](/source/Address_space) simultaneously. ARs were introduced with ESA/370 in 1988, and supported by the [MVS/ESA](/source/MVS/ESA) operating system.[1]

In [IBM System/360 architecture](/source/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](/source/IBM_System/370#Expanding_the_address_space) 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.[2]: 1-1 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](/source/Address_register)

## References

1. Babcocl, Charles (Feb 22, 1988). ["IBM opens data trove to E-model users"](https://books.google.com/books?id=jBT4fczNvWwC&pg=P1). Computerworld. Retrieved Mar 5, 2020.

1. IBM Corporation (August 1988). [*IBM Enterprise Systems Architecture/370 Principles of Operation*](http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/370/princOps/SA22-7200-0_370-ESA_Principles_of_Operation_Aug88.pdf). Retrieved Mar 6, 2020.

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