ACT apricot - Page 274

KEYBOARD DRIVER
7. Queue handlers for data flow to MS-DOS or screen.
The keyboard software contains 3 ring buffers, or queues,
to pass data between various asynchronous software
modules.
These are:
a. Queued data from the keyboard handler to MS-DOS
b. Queued data to the screen handler (local keys from
the keyboard)
c. Queued data from the MicroScreen driver, LED
driver,
Time and Date handler.
a.
The MS-DOS queue:
This consists of three routines, a queue filler, a look-
ahead character checker,
and a queue reader.
The queue filler is called from the main keyboard handler
and places the character in the 80- byte buffer.
The caller also passes a count of how many characters it
wishes to place sequentially in the buffer, if there is not
enough room for the entire string in the buffer, then the
queue filler does not accept the character, sounds the bell,
and returns an error status.
The character look-ahead routine is called from the
MS-DOS BIOS interface routines to provide a keyboard
look-ahead,
and simply returns the next output character in
the queue, without removing it from the queue (if no
character is available it returns a null code).
The queue reader is called by MS-DOS with an address
where a character from the queue should be placed, if no
character is available, the routine waits for one.
h. Local queue:
The local queue filler is called by the keyboard handler to
pass characters
to the screen driver, and operates in much
the same way as the MS-DOS queue handler.