Browse Prior Art Database

Access Protection of the Real-Time Clock Address Space

IP.com Disclosure Number: IPCOM000115544D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Bealkowski, R: AUTHOR [+3]

Abstract

Disclosed is a method to protect access to the Real-Time Clock (RTC) address space in a PowerPC* system.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 94% of the total text.

Access Protection of the Real-Time Clock Address Space

      Disclosed is a method to protect access to the Real-Time Clock
(RTC) address space in a PowerPC* system.

      In a PowerPC system, the RTC chip does not have a provision for
protecting the registers in the RTC address space from being read or
written.  Since the password and other critical data reside in the
RTC address space, the space must be protected.

      The desktop PowerPC system protects addresses 20-3F of the RTC
address space.  It uses two I/O registers, referred to as PRTCT_reg1
and PRTCT_reg2.  PRTCT_reg1 is used to block access to addresses
20-2F of the RTC space.

Note:  The address granularity of the protected space by each latch
may vary depending on the system and the available I/O ports.

The protection sequence for the desktop PowerPC system is as follows:
  1.  Write to PRTCT_reg1.  This sets PRTCT_reg1.
  2.  All subsequent accesses to address 20-2F of the RTC space will
       return "FF" data.

      The same scheme applies to PRTCT_reg2.  The PRTCT_reg registers
are cleared only on power-on-reset.

      The addresses and data must be multiplexed on the data bus to
the RTC chip.  The address is written on port 70 and data on port 71.
If any of the PRTCT_reg registers are set and an access to port 70 is
detected with the address falling within the protected range of the
PRTCT_reg registers, then a latch is set which will block reading or
writing to the...