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July 17, 2002
96
System Design Considerations
6.5.3 Combined BIOS/CMS Parallel ROM Interface
TM5500/TM5800 processors support use of the BIOS parallel ROM memory for Code Morphing Software
(CMS) storage, thus eliminating the need for a 1 MByte serial Flash ROM to store Code Morphing software.
This is accomplished by connecting the processor EPROMA[2:1] signals to the highest-order address bits of
a 2 MByte parallel flash ROM, in the usual way for a system BIOS interface (Xbus or similar). Code Morphing
software is stored in the lower ¾ of the ROM, and the upper ¼ is used for the system BIOS. The processor
controls the EPROMA[2:1] signals during Code Morphing software decompression, and then sets
EPROMA[2:1] to 11b to enable access to the x86 BIOS code.
There is an important security issue arising from the use of a parallel ROM for Code Morphing software that
must be addressed by the system designer. This issue relates to the use of JEDEC flash ROMs that have an
erase-all
command that erases all unprotected sectors in the ROM. This allows the possibility for x86
software (applications or viruses) to erase the entire ROM, including the Code Morphing software portion.
To protect against unauthorized Code Morphing software erasure, the Code Morphing software sectors of the
ROM should be protected by enabling the ROM sector protection feature. The ROM sector protection feature
is enabled by the ROM programming equipment at the OEM manufacturing site.
JEDEC flash ROMs have a temporary sector unprotection method that involves raising the reset pin of the
ROM to +12 V. This method is used for Code Morphing software field upgrades. A pin on the processor (not
accessible by x86 code) is used to control the temporary unprotect ROM feature during the upgrade process.
Future versions of parallel ROMs may allow the
erase-all
feature to be disabled, and some may also support
software programmable protection of sectors. These enhancements eliminate the need for this protection
circuit.
For debugging purposes, retain the serial flash connections to the Transmeta Debug Module (TDM) interface.
The TDM has built-in serial flash and allows developers to boot from the TDM-based flash device.
The schematic on the following page illustrates a combined BIOS and Code Morphing software parallel flash
ROM circuit.
Содержание Crusoe TM5500
Страница 1: ...TM5500 TM5800 System Design Guide July 17 2002...
Страница 6: ...July 17 2002 6 List of Tables...
Страница 8: ...July 17 2002 8 List of Figures...
Страница 50: ...July 17 2002 50 Processor Power Supplies and Power Management...
Страница 110: ...July 17 2002 110 System Design Considerations...
Страница 122: ...July 17 2002 122 System Design Checklists...
Страница 128: ...July 17 2002 128 Serial Write protection PLD Data...
Страница 130: ...July 17 2002 130 Index...