Browse Prior Art Database

Methodology to Dynamically Patch a Personal Computer System ROM Utilizing RAM Image Feature

IP.com Disclosure Number: IPCOM000122593D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 73K

Publishing Venue

IBM

Related People

Higgins, PE: AUTHOR [+4]

Abstract

Typically, personal computer (PC) system patch methodology consisted mainly of programs that intercepted documented basic input/output system (BIOS) interfaces. This interception of calls is effective in most cases; however, not all programs use these documented interfaces to call BIOS. For these programs, this patch methodology is totally ineffective. Ideally the best method is to replace the BIOS code directly. However, in previous PC systems, the BIOS code was contained strictly in read-only memory (ROM). An improved method of patching the system ROM is disclosed herein.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Methodology to Dynamically Patch a Personal Computer System ROM Utilizing
RAM Image Feature

      Typically, personal computer (PC) system patch
methodology consisted mainly of programs that intercepted documented
basic input/output system (BIOS) interfaces.  This interception of
calls is effective in most cases; however, not all programs use these
documented interfaces to call BIOS.  For these programs, this patch
methodology is totally ineffective.  Ideally the best method is to
replace the BIOS code directly.  However, in previous PC systems, the
BIOS code was contained strictly in read-only memory (ROM).  An
improved method of patching the system ROM is disclosed herein.

      ROM circuits are normally slow in accessing information.  To
accommodate the faster system speeds in later PC systems, ROM access
was improved by copying the 128K of the ROM to system RAM, remapping
this RAM to the same addresses as the ROM, and disabling access to
the ROM. This allows the system to access the BIOS in the ROM at full
system speed.

      Once copied to system RAM, this 128K requires protection from
inadvertent modification.  This requires the addition of hardware
that would prevent writes to this area under normal system operation.

      By using this ROM-to-RAM remap feature of the system, a method
is devised of effectively patching the code which was copied from the
ROM.  Patching the ROM directly allows the following:
o    Does not take any of the user's memory.
o    Does not alter the entry points to any of the documented inter
faces.
o    Does not affect the calling interface of user programs.
o    Allows for variable size patches.
o    Allows patches to non-code areas.
o    Does not...