Gidubba v1.0
============

Gidubba (Sumerian for 'stylus') is a simple line editor for the
Thingamajig computer architecture. Its git repository can be found at
https://ahti.space/git/crazyettin/Gidubba and that of Thingamajig at
https://ahti.space/git/crazyettin/Thingamajig.

System requirements
-------------------

Gidubba requires at least 4 KiB of RAM. It uses a terminal with local
echo at address FFFF, and optionally a character printer at address FFFE
and a punched tape reader and punch or a casette drive at FFFD. Do not
be afraid to modify the code yourself if your setup requires it! (And if
you do, apologies for the spaghetti.)

Format
------

Texts are terminated by an end-of-file (^Z) and lines within them by a
newline of a carriage return (^M) followed by a line feed (^J). Note
that the input buffer can store at most 256 characters, including the
newline.

Usage
-----

The commands are individual letters that can be followed by one or two
arguments separated by a comma, all case-insensitive. The first argument
is a line number between 0 and FFFF and the second a range of lines
between 0 and FF. To append lines, insert them or set the mark to any
line number after the text or to the shortcut hash (#); in other
commands line numbers or parts of a range after the text are ignored and
a hash is not recognised as a valid argument. Instead of a predetermined
range a prompt to insert the next line appears automatically after the
previous one has been committed.

Commands with a single argument:
  * I: Insert lines
  * M: Set the mark

Commands with two arguments:
  * C: Copy to the mark
  * D: Delete lines
  * L: List lines

Commands with no arguments:
  * P: Print
  * R: Read from the tape reader or casette drive
  * W: Write to the tape punch or casette drive

A delete (^?) discards the preceding character and outputs an underscore
to the terminal. An escape (^[) discards the contents of the current
command or line and outputs a backslash and a newline to the terminal. A
carriage return (^M) or a line feed (^J) inputs a newline and commits
the current command or line. An end-of-file (^Z) discards the current
command and halts the computer or discards the current line and returns
to the command prompt.

There are two kinds of error messages: a question mark (?) for an
erroneous command and an exclamation mark (!) for lack of RAM.