Chapter 27 - Organization of memory
The memory is sorted into different areas for storing different kinds of information. The areas are only large
enough for the information that they actually contain, & if you insert some more at a given point (for
instance by adding a program line or variable) space is made by shifting up everything above that point.
Conversely, if you delete information then everything above it is shifted down.
The system variables contain various pieces of information that tell the computer what sort of state the
computer is in. They are listed fully in the next chapter, but for the moment note that there are some (called
D_FILE, VARS, E_LINE & so on) that contain the addresses of the boundaries between the various areas
in the memory. These are not BASIC variables, & their names will not be recognized by the computer.
Each line in the program is stored as:
Note that, in contrast with all other cases of two-byte numbers in the Z80, the line number here (& also in
a
FOR-NEXT
control variable) is stored with its most significant byte first: that is to say, in the order that
you would write them down in.
A numerical constant in the program is followed by its binary form, using the character
CHR$
126
followed by five bytes for the number itself.
The display file is the memory copy of the television picture. It begins with a
NEWLINE
character, & then
has the twenty four lines of text, each finishing with a
NEWLINE
. The system is so designed that a line of
text does not need space a full thirty two characters: final spaces can be omitted. This is used to save
space when the memory is small.
When the total amount of memory (according to the system variable RAMTOP) is less than 3 1/4 K, then
a clear screen - as set up at the start or by
CLS
- consists of just twenty five
NEWLINE
s. When the
memory is bigger than a clear screen is padded out with 24*32 spaces & on the whole it stays at its full
size;
SCROLL
, however, & certain conditions where the lower part of the screen expands to more than two
lines, can upset this by introducing short lines at the bottom.
The variables have different formats according to their different natures.
Number whose name is one letter only:
To Next Page
Loading...
Need help?
General
