Commodore Amiga Hardware Reference Manual page 61

DIWSTRT EQU $DFF08E
MOVE.W #$2081,DIWSTRT
Setting the Display Window Stopping Position
jDisplay window start
; register address
j
Write it out
You also need to set the display window stopping position, which is the lower right-hand
corner of the display window. If you select high-resolution or interlaced mode, the stop-
ping position does not change. Like the starting position, it is interpreted in low-
resolution, non-interlaced mode.
The register DIWSTOP (for Display Window Stop) controls the display window stopping
position. This register contains both the horizontal and vertical components of the
display window stopping positions, known respectively as HSTOP and VSTOP. The
instructions below show how to set HSTOP and VSTOP for the basic playfield, assum-
ing a starting position of ($81,$20). Note that the HSTOP value you write is the actual
value minus 256 ($100). The HSTOP position is restricted to the right-hand side of the
screen. The normal HSTOP value is ($101) but is written as ($01).
The VSTOP position is restricted to the lower half of the screen. This is accomplished
in the hardware by forcing the MSB of the stop position to be the complement of the
next MSB. This allows for a VSTOP position greater than 256 ($100) using only 8 bits.
Normally, the VSTOP is set to ($F4).
The normal DIWSTRT is ($2081). The normal DIWSTOP is ($F401).
The following example sets DIWSTOP for a basic playfield to $F4 for the vertical posi-
tion and $01 for the horizontal position.
DIWSTOP EQU
$DFF090
MOVE.W #$F401,DIWSTOP
48 Playfield Hardware
;Display window stop
; register address
; Write it out