Atari ST series Technical Reference Manual page 13

The ST Operating System: An Overview
Line A routines
Exception Handlers
Startup Code
communicating with the character de­
vices like the screen, keyboard,
printer, and serial port.
Line A routines are the low-level
graphics routines the GEM VDI calls
for basic graphics functions. These
functions include setting and reading
individual pixels, drawing lines and
filled polygons, and moving software
sprites, like the mouse pointer. Since
the ST screen is bitmapped, the line A
routines are also used for drawing all
text characters on the screen. Using
the line A routines for graphics and
text provides greater compatibility
than accessing the ST graphics hard­
ware directly, because such programs
will continue to function correctly
even if the ST graphics hardware
changes. For example, programs that
use the line A routines can take ad­
vantage of the blitter chip used by
later ST models, while programs that
write to screen memory directly can­
not.
Many of the Operating System rou­
tines are invoked by interrupts and
trap instructions, which in 68000 par­
lance are referred to as exceptions. For
example, the BIOS routines are called
via the TRAP #13 instruction, GEM­
DOS routines via the TRAP #1 in­
struction, and GEM AES and VDI rou­
tines are called via the TRAP # 2
instruction. In addition to the han­
dlers that route these calls, there are a
number of lower-level interrupt han­
dlers, such as the vertical blank inter­
rupt handler, which are of interest to
programmers.
The startup code is a short piece of
program code that is called when the
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