When the editor receives a read
character function call, it examines the EXOS variable
FLG_EDIT. This byte contains a series of flags which control
the response of the editor to this read character function call. The
editor's action will first be described in general terms without
reference to the individual flags. The effect of each flag will then be
described in detail.
Assuming for now a typical setting of the flags in FLG_EDIT,
when the editor receives a read
character function call it interprets this as a request to send a
line of text back to the applications program. It does not return a
character immediately but records the state of the flags and enters its
main editing loop which provides the actual editing facility to the user.
It is only while in this loop that the cursor on the video page will
flash. A flashing cursor thus indicates that the editor is waiting for a
key to be pressed.
This loop reads a character from the keyboard, responds to it and then loops back for another one. This allows the user to type in text which will be inserted into the editor's text buffer and displayed on the video page, and to carry out various editing functions. The details of the editing functions are given later. There are two keys which are of importance here - Escape and Enter.
If Escape (ASCII code 01Bh) is received from the
keyboard then this character will immediately be returned to the
applications program as the response to the read character call,
regardless of the state of any of the FLG_EDIT flags. This
provides a way of interrupting a line input operation.
If Enter (ASCII code 0Dh) is pressed then this is a
command to the editor to begin sending text back to the applications
program. The details of how much text is sent are determined by the flags
and will be described below. An internal editor flag is set to indicate
that it is in the process of sending text, and the first character is
returned to the applications program. When the applications program makes
another read character call the internal flag is still set, so instead of
entering the editing loop, it simply returns the next character of the
requested text immediately. This continues until all the required text
has been sent at which time the internal flag is cleared so the next read
character call will again enter the editing loop.
It is up to the applications program to recognize when it has read all the characters to avoid restarting a new read operation. How to recognize this depends on the setting of the editor flags and is described later.
Note that the editor flags are sampled once when the first read character call is made and then not again until all the text has been sent. Changing them in the meantime will therefore have no effect on the read which is in progress. If a write character call is made while a read is in progress then the internal flag is cleared so that the read will be aborted. This also applies if any special function calls are made.
The assignment of bits in the FLG_EDIT EXOS variable is:
bit7 - SEND NOW
bit6 - SEND ALL
bit5 - NO READ
bit4 - NO SOFT
bit3 - NO PROMPT
bit2 - AUTO ERA
bit1 - not used
bit0 - not used
SEND NOW Flag (bit7)
If this flag is set then the editor will start returning text immediately, without reading from the keyboard at all. If it is clear then the main editing loop will be entered and no text will be returned until the user types Enter. In either case the amount of text returned is the same and depends on the setting of the other flags.
SEND ALL Flag (bit6)
This is the main flag which determines how much text is sent. If it is clear then the paragraph containing the cursor will be sent. If it is set then the whole editor buffer will be sent.
In the first case the applications program will be sent the characters of
the paragraph one by one terminated with a CR and then an
LF. This CR-LF can be used by the applications
program to determine when to stop reading (beware if
NO SOFT is clear! - see below). The
cursor will be left on the first character of the next paragraph with a
new (empty) paragraph being created if that was the last one.
If SEND ALL is set then the entire buffer will be sent. This
will include CR-LF sequences at least between each paragraph
(again see NO SOFT flag below) so this
cannot be used to indicate the end of the text. Instead, after the last
CR-LF has been sent (the last characters sent will always be
CR-LF), the next character read will produce a
.EOF (end of file) error. This error will only be received
for one character so the applications program must notice it and stop
reading. If another character was to be read the this would start the
whole reading process again from the start of the buffer.
NO READ Flag (bit5)
If this flag is set then Enter will not in fact return any
characters at all to the applications program. The only way to get back
to the applications program is thus to press Escape. Although no
characters are returned, the routine which selects which characters to
send depending on the flags, is still executed. Thus the cursor is moved
in the same way as if the text was returned. If SEND
ALL is clear then pressing Enter will just move to
the start of the next paragraph, putting in a new line if it was at the
end of the buffer. This will make the editor behave rather like a
typewriter.
NO SOFT Flag (bit4)
This flag controls the sending of soft carriage returns and soft spaces. Soft carriage returns are those which separate successive lines of a paragraph. Soft spaces are those spaces which are inserted for justification, and also the spaces before the left margin of a line.
If this flag is clear the soft spaces are returned as normal ASCII spaces
(20h) and soft carriage returns are returned as normal
CR-LF sequences. If the flag is set the both of these are
suppressed and no characters are returned for them.
Beware that if NO SOFT and SEND
ALL are both clear then there is no way for the applications
program wether a CR-LF which it receives is the end of the
paragraph, in which case it should stop reading, or simply the separator
between two lines of the paragraph, in which case it should continue.
Therefore this combination of flags should be avoided - at least one of
them should always be set.
NO PROMPT Flag (bit3)
This flag is normally clear. If it is set then the cursor position when
the read operation was started is remembered. When it comes to returning
text, if the cursor has not been moved out of the original paragraph, or
to before the remembered postition in the current paragraph, and if
SEND ALL is clear, then the current
paragraph will be sent back but starting from the character at the
remembered cursor position rather than the start.
If the cursor has been moved out of these bounds then the whole paragraph
will be returned as usual, or the whole editor buffer if
SEND ALL is set
This feature is used by BASIC when doing an INPUT command.
It prints the prompt and then does an editor read with this flag set.
When the paragraph is sent to BASIC the prompt will not be returned, just
the response to it.
AUTO ERA Flag (bit2)
This flag is also normally clear. If it is set then, if the very first
character typed at the keyboard is a printing character (as opposed to an
editing function or cursor movement) then the current line will be
cleared before responding to the key. This is provided mainly for BASIC
to allow commands such as RUN to be typed on top of an
existing lines after editing a program. It may just conceivably be of
some use to other applications programs.
The use of these flags can be rather confusing so this section discuses some examples of their use from ISBASIC and the built in word processor (WP). This covers most useful combinations and certainly shows the use of each of the flags.
Flags=000101xx
When BASIC is reading a command line from the editor it is expecting
either an immediate mode command (such as RUN) or a new line
starting with a line number to be typed. In either case it wants to read
a single paragraph entered by the user. This might be newly typed by him
or it might be a line which already exists in the editor buffer which he
simply moves the cursor to and reenters.
Clearly BASIC wants to wait for the user to type Enter so the
SEND NOW flag is clear. Only one
paragraph, rather than the whole buffer is wanted so
SEND ALL is clear and BASIC wants to
actually be sent the text so NO READ is
clear. BASIC is not interested in the breaks between lines in the
paragraph (since one command line can over flow onto several screen
lines) so NO SOFT is set.
NO PROMPT is clear and
AUTO ERA is set (as explained above).
Flags=000110xx
When BASIC is doing an input command it also wants to read in a single
paragraph so the SEND NOW,
SEND ALL, NO
READ and NO SOFT flags are all
set the same as for reading a command. In this case however BASIC will
write out a prompt and wants to read in just the response to that prompt,
rather than having the prompt at the start of the paragraph which it
receives, To do this it sets the NO
PROMPT flag. The AUTO ERA flag
is clear.
Flags=001xx0xx
In normal editing mode the word processor wants to let the user get on
with his editing without data being sent to the word processor. The
SEND NOW flag is clear to allow the user
to do editing. The SEND ALL is clear and
the NO READ is set to ensure that no
characters are returned to the word processor when Enter is
pressed but the cursor will be moved to the start of the next paragraph.
The NO SOFT and NO
PROMPT flags are irrelevant when NO
READ is set, and the AUTO ERA
flag is clear.
This will have the effect of allowing the user to move all over his
document, pressing Enter and using any of the editing features.
Only when Escape is pressed will the word processor applications
program be alerted. In fact the eight function keys have the
ESC code as the first code in their programmed string so the
word processor gets alerted when they are pressed as well.
Flags=11000xxx
When the word processor is asked to print a document it must read the
wole of the editor's text buffer in order to print it. Also it wants to
get the data immediately rather than waiting for the user to press
Enter. To achieve this SEND NOW
and SEND ALL are both set.
NO READ is of course clear or no text
would be sent and NO SOFT is clear so
that all spaces and soft carriage returns will be sent to ensure that the
formatting of the printed document is correct. NO
PROMPT is clear. AUTO ERA does
not matter because SEND NOW is set so the
user doesn't get a chance to press a key.