Browse Prior Art Database

Scheme of Read Only Memory Emulation of MAC OS

IP.com Disclosure Number: IPCOM000118957D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Hoang, B: AUTHOR [+4]

Abstract

Disclosed is a scheme of Read Only Memory (ROM) emulation or ROM-less design in a ROM-based Mac* OS. This design lowers the system cost while allowing the CPU local bus to run faster due to shorter signal lines and better signal quality.

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Scheme of Read Only Memory Emulation of MAC OS

      Disclosed is a scheme of Read Only Memory (ROM) emulation or
ROM-less design in a ROM-based Mac* OS.  This design lowers the
system cost while allowing the CPU local bus to run faster due to
shorter signal  lines and better signal quality.

      The North bridge of a PowerPC** Reference Design provides a ROM
shadowing capability.  ROM shadowing is a method that ROM addresses
are mapped (on-the-fly) to DRAM address space.  This speeds up the
execution of the ROM program but the shadowing still requires a Mac
ROM over the  CPU bus.

      A ROM-less design expands the idea of shadowing; before
loading the Mac OS, firmware locates the ROM image from the boot
device.  Firmware copies ROM image to top 4 MB of system DRAM.
Firmware enables the shadow mode so future references to ROM access
the top 4M of  of DRAM.  Firmware then reports available memory equal
to the actual memory minus 4 MB.  After these steps, Mac OS is
loaded.  Mac OS executes  a ROM image just like a physical ROM is
there.
   *  Trademark of Apple Computer, Inc.
  **  Trademark of IBM Corp.