Front Panel Load Address Operation

The way the data paths are set up, the easiest way to get a value from the switches to the PC is to go through the instruction register. However, I can't load the whole instruction register in the middle of a microcode sequence, because that would mess up the microcode address. So, I have split out the address part of the instruction register into a Memory Address register that I can load separately.

A quirk of this approach is that the value to be loaded into the PC needs to be set into switches 2 to 11, not 1 to 10 as might have been expected. This is due to the fact that IR/MA is loaded with an instruction during T17 of a fetch cycle, at which time the final shift of the value in the S register has not yet been performed; to compensate for this, inputs 0 to 16 of IR/MA are wired to outputs 1 to 17 of the S register.

This is not really a problem, it's just a matter of labelling the switches appropriately. I created a bezel for the switches on the main schematic showing where the hex digits go for PC values and data.

Load Address Microcode

# MISC values
LDMA  = 1101  # Load MA register from S register

# Load Address
   1 00001 0 0000 :  -   -   --  ---   -  --    -     -   --   -   -   -   -   -    -   PLS     -   ---    PISW -- # Load switches into S
   1 00001 0 0001 :  -   -   --  ---   -  --    -     -   --   -   -   -   -   -    -   LDMA    -   ---    ----    # Load MA from S
   1 00001 0 0010 :  -   -   --  ---   -  --    -     -   --   -   -   -   -   -    -   LDPC    -   ---    ----    # Load PC from MA
   1 00001 0 0011 :  -  EOI  --  ---   -  --    -     -   --   -   -   -   -   -    -   HALT    -   ---    ----    # Stop