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A251: Setting the Address of Config Bytes

Information in this article applies to:

  • C251 All Versions


I'm using the 251 microcontroller with a 27C256 (32K EPROM) and I need to know where I should locate the configuration bytes in the START251.A51 startup code file.


The configuration bytes are located at FF:FFF8h and FF:FFF9h. However, since the 32K EPROM does not extend that high in memory, the config bytes must be located at the addresses read when FF:FFF8h and FF:FFF9h are asserted on the address bus.

In the 32K EPROM, these addresses are 7FF8h and 7FF9h. The code in START251.A51 should appear as follows:

CONFIG0         EQU     (WSA*20H)+(XALE*10H)+(RDRG*4)+(PAGM*2)+SRCM+080H
CONFIG1         EQU     (INTR*10H)+(EDF*8)+(WSB*2)+EMAP+0E0H

                CSEG    AT      07FF8H
                DB      CONFIG0         ; Config Byte 0
                DB      CONFIG1         ; Config Byte 1


Last Reviewed: Wednesday, September 21, 2016

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