Table of Content  Branch Up  

Peripherals

We call "peripheral" to any device that is external to the CPU. Included in this group are: the Main Memory, the COM Ports, the Tape Drive Controllers and many others. All of them are mapped within the same addressable space.

Some peripherals (notably, Terminals) connects to the CPU via RS232 ports. Most others, however, are attached to a common external bus that we call "U-BUS" consisting of 16 data lines, 16 address lines and 7 control lines.

Peripherals contain their own memory chips allocated in designated addresses. These are wired as dual-port memory so both the CPU and the peripheral circuitry can read/write from/to them. Typically, a status register and at least one-word data buffer are implemented per peripheral.

A typical communication between the CPU and a device runs like this:

The CPU sends a command to the device by writing the device's status register. For the CPU prospective it only had wrote a word to a memory cell. The CPU does not wait for the response but it keeps working on something else.

The Device reacts to the command doing some work. Say for instance that it gets data from the magnetic tape to fill in its internal Buffer.

When the Device finishes, it interrupts the CPU which, in response, reads the Device Buffer into Main Memory. Possibly this cycle repeats several times until the desired amount of data is read into Main Memory.