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Routines |
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Check that an object isn't moving (e.g. carried by the servant) below the bottom of the room boundary.
Recalculated vertical pixel position returned in E register, ready for a later call to the room border checks at E039
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| DAC8 | LD A,($EB4A) | Graphic vertical position (of object) from top of screen (pixels) | ||
| DACB | LD E,A | Store in E register | ||
| DACC | LD HL,$EAF6 | Address holding vertical position of room (viewport) in pixels | ||
| DACF | INC HL | ...the main value is stored in the high byte of the address, so advance to this address | ||
| DAD0 | LD A,$60 | Offset (96 pixels, from top of screen) for the bottom of the room's active playing area | ||
| DAD2 | ADD A,(HL) | Add to the room's vertical position (e.g. if Maroc is towards the top of the room, and the room extends downwards off the bottom of the playing area, then it's OK for the object to be dragged there)... | ||
| DAD3 | SUB E | ...Subtract the object's vertical screen position | ||
| DAD4 | JR C,$DADB | If this results in a carry, it's too far off the bottom of the playing area | ||
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While the servant can move anywhere in the visible screen area, objects are constrained by room wall boundaries.
The next set of instructions adds a small offset (based on the room offset position) to the item's vertical position, which is used for later room boundary checks. i.e. extends the boundary
This lets the servant drag the object into the room's wall areas - to an extent at least. Without this adjustment it would be difficult to do certain things, such as drag objects to trade with warlocks.
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| DAD6 | RRA | The A register still contains (from the previous instructions) the difference between the item's vertical position and the bottom of the screen area | ||
| DAD7 | AND $7F | Halve this value, and filter out bit 7 as it's not needed | ||
| DAD9 | ADD A,E | Store in E register | ||
| DADA | LD E,A | |||
| DADB | RET | |||
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