jetpac: Routine at 72b6

72b6: Calculate screen address one pixel above current position.

Calculates the new address for writing a sprite pixel, in an upward direction, taking into consideration the screen memory layout.

Used by the routine at 7705.

Input
HL	Current position.

Output
HL	Address for new position.

72b6

dec h

Decrement H to move up one pixel on screen

72b7

ld a,h

72b8

and $07

72ba

cp $07

Has a character line been crossed?

72bc

ret nz

If not, return

72bd

ld a,l

else subtract 32 from L

72be

sub $20

72c0

ld l,a

72c1

and $e0

72c3

cp $e0

Has a new section of the screen been crossed?

72c5

ret z

Return if not

72c6

ld a,h

72c7

add a,$08

72c9

ld h,a

72ca

ret