Atari ST series Technical Reference Manual page 54

Chapter 5) and store its address in location $502. Since that
location is in protected memory, first switch to Supervisor
mode (see the Supexec() function below). If you want to
keep the new driver installed after the program ends, use
GEMDOS function 49 ($31) to keep the program code resi­
dent when it terminates.
The screen print vector at $502 can be diverted for other
purposes in addition to installing new printer drivers. Some
snapshot programs, for example, use this vector to install a
routine that saves the screen picture to a disk file when the
Alt-Help keys are pressed, rather than sending it to a
printer. It's also possible to install a short routine that tests
for shift-keys when Alt-Help is pressed, This allows addi­
tional hot-key programs to be installed, rather than just re­
placing the screen print function.
The default screen print code calls another XBIOS func­
tion to do the actual printing. This function can be used to
print all of the screen or only a part of it. Its name is
Prtblk(), and it's called like this:
long prtable;
Prtblk(prtable);
where prtable contains the address of a 30-byte parameter ta­
ble that determines how the screen block is printed. The
composition of this table is as follows:
Table 3-7. Structure of Table Pointed to by prtable
Byte Element
Number Name
0 -3 blkprt
4 -5 offset
6 -7 width
8 -9 height
10-11 left
right
12-13
14-15 scrres
16-17 dstres
18-21 colpal
46
CHAPTER 3
Description
Starting address of screen RAM
Offset from start address (in bits, 0-7)
Screen width (in bytes)
Screen height
Left margin for screen dump
Right margin for screen dump
Screen resolution
0 = Low
1 = Medium
2 = High
Printer resolution
0 = Draft (960 dpi)
1 = Final (1280 dpi)
Starting address of the color palette