64K RAM on Altair FDC+
Posted: June 12th, 2020, 11:43 am
Because of the way the DIP switches are configured on the Altair FDC+, the top 256 bytes of RAM are not available (FF00-FFFF). If this is a problem for something you may be working on, a very simple mod to the FDC+ allows RAM through FFFF.
1) Cut the trace between U9p13 and U8p19
2) Connect U9p13 to U9p14
Prior to the mod, the page address specified by the PROM switches was the start page of the EPROM and last page+1 of RAM. For example, if page FC was specified on the PROM switches, then the EPROM started at FC00 and the last page of RAM was FB (FB00-FBFF).
With the mod, the page address specified by the PROM switches is the start page-1 of the EPROM and the last page of RAM. For example, if page FC is specified on the PROM switches, then the EPROM starts at FD00 and the last page of RAM is FC (FC00-FCFF).
This means that if the PROM switches specify page FF, then the last page of RAM is FF (FF00-FFFF) and a full 64K of RAM can be enabled. In this case, the EPROM is not accessible and the PROM enable switch should be set to "off."
Mike
1) Cut the trace between U9p13 and U8p19
2) Connect U9p13 to U9p14
Prior to the mod, the page address specified by the PROM switches was the start page of the EPROM and last page+1 of RAM. For example, if page FC was specified on the PROM switches, then the EPROM started at FC00 and the last page of RAM was FB (FB00-FBFF).
With the mod, the page address specified by the PROM switches is the start page-1 of the EPROM and the last page of RAM. For example, if page FC is specified on the PROM switches, then the EPROM starts at FD00 and the last page of RAM is FC (FC00-FCFF).
This means that if the PROM switches specify page FF, then the last page of RAM is FF (FF00-FFFF) and a full 64K of RAM can be enabled. In this case, the EPROM is not accessible and the PROM enable switch should be set to "off."
Mike