Routines
Prev: A80B Up: Map Next: A8D9
A8A5: Add To Stack
Used by the routine at DrawShot.
Adds one or two units to the height of the stack at StackHeight (which hangs below the ship) if the item that is carried at F601 is item number 06. The limit is 12. The screen is cleared where the new units will be drawn. This is a best reading of the code. An object with a state of zero does not add anything.
Input
IX Address of the object
AddToStack A8A5 LD A,(IX+$13) Fetch the state of the object.
A8A8 AND A Return if it is zero.
A8A9 RET Z
This entry point is used by the routine at CheckShotCollisions.
The routine is entered here with a state in A when the number of units is random.
AddToStack_Units A8AA AND %00000010 Keep bit 1 of the state...
A8AC SRL A ...move it to bit 0...
A8AE INC A ...and add one to get one or two units.
A8AF LD B,A Store the number of units in B.
A8B0 LD A,($F601) Fetch the item that is carried.
A8B3 CP $06 Return if it is not item number 06.
A8B5 RET NZ
A8B6 LD A,($C905) Fetch the height of the stack and keep the old height in E.
A8B9 LD E,A
A8BA ADD A,B Add the new units.
A8BB CP $12 Jump to AddToStack_Store if the height is below the limit of 12.
A8BD JR C,AddToStack_Store
A8BF LD A,$12 Use the limit.
AddToStack_Store A8C1 LD ($C905),A Store the new height of the stack.
A8C4 LD A,($C903) Fetch the vertical position of the ship.
A8C7 SUB E Subtract the old height to find the first new row.
A8C8 LD H,A Store it in H.
A8C9 LD L,$70 Use the horizontal position of the stack.
A8CB CALL GetScreenAddress Find the screen address.
AddToStack_Clear A8CE LD (HL),$00 Clear the left byte of the row.
A8D0 INC L Move on to the right.
A8D1 LD (HL),$00 Clear the right byte of the row.
A8D3 DEC L Move back to the left.
A8D4 CALL MoveUpARowDE Move up a row.
A8D7 DJNZ AddToStack_Clear Loop back to AddToStack_Clear for each new unit.
Continue on to BuildStackGraphic.
Prev: A80B Up: Map Next: A8D9