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Copy Carry to Bank Address Register Instruction

IP.com Disclosure Number: IPCOM000060788D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Marquart, DW: AUTHOR

Abstract

A control store processor (CSP) such as the one used in IBM System/36 normally accesses control store by word rather than by byte. Byte accessing capability, however, is useful when manipulating character strings, arrays of byte data, and such data as display screen formats and images and blocks of data to be transmitted or received from a communications device. The CSP includes a feature called the Bank Address Register (the BAR bit), which is a single-bit register. This bit allows access to the high-order half of storage (the upper 65,536 bytes) in byte addressing mode by providing the high-order bit of the address. The CSP can address 65,536 16-bit words which are equal to 131,072 bytes. The 16-bit addressing structure of the CSP can not address the higher 65,536 bytes of control store.

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Copy Carry to Bank Address Register Instruction

A control store processor (CSP) such as the one used in IBM System/36 normally accesses control store by word rather than by byte. Byte accessing capability, however, is useful when manipulating character strings, arrays of byte data, and such data as display screen formats and images and blocks of data to be transmitted or received from a communications device. The CSP includes a feature called the Bank Address Register (the BAR bit), which is a single-bit register. This bit allows access to the high-order half of storage (the upper 65,536 bytes) in byte addressing mode by providing the high-order bit of the address. The CSP can address 65,536 16-bit words which are equal to 131,072 bytes. The 16-bit addressing structure of the CSP can not address the higher 65,536 bytes of control store. To use the BAR bit, the byte address is shifted right one bit and the shifted-out bit saved, while the BAR bit is shifted into the high- order bit of the address. The resulting address is used as a word-type address in control store; the low-order bit that was shifted out and saved is used to determine which byte of the storage word thus addressed is to be used (it is the "byte selection bit"). On reads from control store, the word is read and the proper byte selected (by using the byte selection bit) to be loaded into the selected register byte. On writes to control store, the addressed word is read into a temporary register, the proper byte (selected by using the byte selection bit) copied over with the data in the selected register byte, and the word is re-written using the word address that was constructed to read the word originally. The problem with this arrangement is that the BAR bit is astatic element of the CSP which makes processing byte structures difficult. The most common method of using the byte addressing feature is to create byte addresses from a word address. The user must set the BAR bit from a properly initialized register each time it is desired...