Micro­power

been stored at £0C78 – £0C79, commands U and N can be used from the keyboard, and DF 55 and DF 4E can be used within programs, to turn the printer on and off.

OUTPUT O xx yy

This routine sends data yy to port xx. The port number, xx, is placed on the bottom eight address lines, A0 – A7, and this is decoded to determine which input/​output device is activated. The data to be sent, yy, is placed on the data bus, and the activated device receives it. The main use of the output command is to communicate with external devices via the PIO chip, so perhaps a few words on the operation of this device would not be out of place here.

The MK3881 PIO chip used on the Nascom is a programmable input/​output chip which the main processor sees as four ports; of these, two ports each provide eight lines, which can be programmed to be input, output or bidirectional, for external communication, while the remaining two ports are used to control the operation of the chip. On the basic Nascom port A is addressed as port 4, and it is controlled by port 6; port B is addressed as port 5 and its control port is port 7. A convenient way to study the operation of the PIO chip is to use the 0 and Q commands to write to and read from the PIO, while monitoring the state of the lines with a logic probe, or with the Bits and P.C.s port probe.

PREGS

In the Nas-Sys 1 monitor command P merely produces an error message; in Nas-Sys 3 the command prints out the contents of the procesors main registers, previously stored in the workspace from £0C61 to £0C6C, together with the current contents of the I, IX and IY registers. Virtually the same code is used to display the registers in Nas-Sys 1, although the format of the display is slightly different (see the S command), but it is not written as a subroutine, so it cannot be accessed from the keyboard or from user programs.

QUERY Q xx

Q xx obtains data from port xx and displays it on the screen in hexadecimal format. To be able to obtain data via the PIO chip you will have to program the chip by writing to the appropriate control port using the 0 command.

READ R xxxx (y)

The Read command loads data from a cassette tape written in the format used by the standard Write command. After turning on the tape LED the routine sets the input/​output table addresses to their ‘normal’ values, saving the addresses that were at £0C73, £0C75 on the stack, so that they can be restored