The Mini-Altair

Version 4

Version 4 expands the RAM from 32K to 64K.  This allows versions of CP/M that are configured for more than 32K of RAM to run.

Theory of Operation

RAM chips

To avoid adding another socket and more wiring, the second 32K RAM chip is just stacked on top of the first chip.  All the pins are soldered in parallel, except the chip select pins.  The address decoding logic for the RAM was modified to provide separate chip select signals for each chip depending on the state of A15.

Schematic

Note the schematic does not show all the power supply and ground connections to the chips or all the bypass capacitors. 

CP/M disk image

The CP/M from the Altair32 software package was used.  Trying to boot using the CP/M disk images in the package results in the system hanging after outputting the welcome message.  Examining the CBIOS source code file provided in the package shows that the problem is caused by the BIOS resetting the console port after it outputs the welcome message.  Sending the reset byte to the UART resets the baud rate clock divider and the number of stop bits and should be followed by sending another byte that initializes these to the correct values.  But only the reset byte is sent, so the UART doesn't function after this point.  The solution is to NOP out the reset instruction bytes.  This requires modifying the disk image.  The reset bytes are NOP'ed out and the welcome message is changed.  The ROM disk bootloader checks a checksum byte for each sector it reads, so these bytes must also be changed for the modified image to boot.