Output Hardware Modification

I made a small change to the output interface. I realised it was probably a bad idea to connect the OBSY signal from the output device directly to the ubranch multiplexer, because glitches could occur if it changes near a clock edge, so I added a flip-flop to synchronise it with the bit clock.

Input Hardware

The Input instruction reads a character from an input device (originally a paper tape reader) and stores it in the lower 5 bits of a memory location. To support this, I designed the following input interface. It consists of a 5-bit parallel-in serial-out shift register, and some logic to manage a pair of handshaking signals. INPUT RDY is asserted by the input device when there is data present, and INPUT ACK is pulsed by the CPU once the data has been accepted. The device is then expected to remove INPUT RDY and start reading the next character.


Reading the input data is a two-step process: first the data is loaded into the input register in parallel, then it is shifted out serially and written to memory. Rather than use up two MISC values for this, I allocated one MISC value called INPT and used WMEM to distinguish between loading and shifting. This effectively gives me two control signals for the price of one.

The INPUT RDY signal is synchronised with RUNCLK to produce IRDY, which is connected to an input of the microcode branch multiplexer. The serial output from the input register is connected to an input of the X multiplexer.

For testing purposes, I connected INPUT RDY to a flip-flop that emulates the handshaking action of an input device.

Other changes to the main circuit:

Input Microcode

INPT  = 011  # Load/shift input register depending on WMEM

BIBSY = 011  # Input data not available

# I - Input
   01000 0 0001 :  -   -   --  ---   -  --    --   --   -   -   -   -   -    -    -   ---     -   BIBSY  1000    # Loop until input data available
   01000 0 0010 :  -   -   --  ---   -  --    --   --   -   -   -   -   -    -    -   INPT    -   ---    ----    # Load data into input register
   01000 0 0011 :  -  EOI  --  XIN   -  --    --   --   -   -   -   -   -    -   WMEM INPT    -   ---    ----    # Write input register to memory