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Method of Expanding the Available Memory Space in Writeable Computer Cartridges

IP.com Disclosure Number: IPCOM000061320D
Original Publication Date: 1986-Jul-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Piazza, WJ: AUTHOR

Abstract

This article describes a method of placing a large amount of ROM (read-only memory), RAM (random-access memory) or Non-Volatile RAM (NVRAM) memory into a computer cartridge in such a way that it does not exceed the architectural limits of the cartridge memory map but allows all of the memory in the cartridge to be accessed when it is needed. This is accomplished by a "paging" scheme which allows software to select by writing to otherwise wasted memory locations the section of memory which should be active. In a system which includes slots for two cartridges which are writeable, this means that read and write lines are available at the cartridge connector so that the designer of a cartridge may include NVRAM or ROM within the cartridge.

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Method of Expanding the Available Memory Space in Writeable Computer Cartridges

This article describes a method of placing a large amount of ROM (read-only memory), RAM (random-access memory) or Non-Volatile RAM (NVRAM) memory into a computer cartridge in such a way that it does not exceed the architectural limits of the cartridge memory map but allows all of the memory in the cartridge to be accessed when it is needed. This is accomplished by a "paging" scheme which allows software to select by writing to otherwise wasted memory locations the section of memory which should be active. In a system which includes slots for two cartridges which are writeable, this means that read and write lines are available at the cartridge connector so that the designer of a cartridge may include NVRAM or ROM within the cartridge. Other signals available at the cartridge connector which are significant are data bus lines, chip select lines (which are derived from some address lines), and the remainder of the address lines (those not used to derive chip selects). Because the processor driving this system does not have an infinite address space, the chip select lines available at the cartridge connector limit the cartridge to holding 128K of memory. The disclosed herein method overcomes this limitation without violating any architectural groundrules. Throughout this disclosure the term memory refers to any type of memory (ROM, RAM, NVRAM, etc.), unless specifically stated otherwise. The technique involves supplying the cartridge with additional memory (beyond what will fit within the 128K bytes available for the cartridge) and some control logic. The memory is paged in such a way that while a certain portion of the memory is active (and thus shows up in the memory map), some other portion is inactive. The additional logic provided within the cartridge controls the paging and thus determines which portions of memory are active and which are not. For the technique to be useful, the paging scheme must be under software control. In the technique described here, this is accomplished by writing to the ROM within the cartridge. The write operation is ignored by the ROM, but the additional logic within the cartridge detects the write operation which is addressed to ROM and uses the data being written to set a new page of memory. For this technique to be used then, at least some portion of the 128K bytes available for use by the cartridge must be ROM. The rest of the cartridge address space may be any type of memory. Using only the chip selects provided by the system through the cartridge connector, a granularity of 32K could be obtained (i.e., any 32K bytes of the cartridge memory could be paged out and replaced with another 32K bytes of memory). If the designer wanted to, he could provide additional logic within the cartridge to provide just about any granularity that was desired. It is not necessary that the memory that gets paged in be of the same type a...