Micro­power

The enter key has a very important function; it tells the computer to scan the line containing the cursor and carry out any legitimate command on that line. However, computers tend to be very fussy about the commands they accept; if you put the command in the wrong place, or make a mistake in spelling or punctuation, you will upset it and it will come back at you with its favourite message – Error.

Screen Layout

The Nascom uses a memory mapped display. This means that each position at which a character can appear on the screen corresponds to a location in a special region of the computers memory, often referred to as the VDU RAM (the term ‘RAM’, and acronym for Acc ess Memory dating from the early days of computing, is now used for any type of memory that can be modified directly by the processor during the running of a program). To produce a character on the screen you merely have to put the appropriate code into the correct location. Special circuitry scans the display memory continuously and converts the data it finds there into a signal which writes the characters on a TV screen. You can change the contents of the display memory directly from the keyboard and watch the characters appear. Clear the screen by typing shift/​backspace, and then enter M9E3, i.e., type M9E3 and then press ‘enter’. Make sure that the letter M is on the far left of the screen, otherwise the computer will ignore it. If you have entered the command to the computers satisfaction the cursor will jump to the next line down the screen where 09E3 20 will be printed, with the cursor flashing at the number 2. This shows that the current content of memory location 09E3, which represents the middle of the screen, is 20 – the code for a space. If you now type 07, when you press ‘enter’ this character replaces the space and a small bell-shaped figure with two legs projecting downwards appears in the middle of the screen. You will have noticed that with this method of entry the position at which the change occurs is independent of the cursor location.

The screen can hold 16 lines of 48 characters. The top line is special; you will find that you cannot move the cursor onto the top line by means of the control keys. Of course, you can put data directly into the memory locations corresponding to the top line; if you modify the contents of location BE3 to 07 the little bell will appear in the middle of the top line. If you try to move the cursor off the bottom of the screen, you will find that the screen ‘scrolls’, that is, the cursor stays on the bottom line but all the data on the screen is moved up one line. However, the contents of the top line are unchanged, and the data in the second line is lost. The top line is used for headings, program names, etc. The Nascom screen is not mapped to the display memory in a straighforward way; the start of the memory corresponds to the second line of the display, it runs down to the bottom line, and it is then followed by the top line. It is thus usual to refer to the scrolling part of the display as lines 1 – 15, and the top line as line 16, which can cause some confusion until you become used to it.

Getting Basic

The microprocessor which controls the operation of your Nascom is an electrical device, and it only responds to the patterns of electricity known as