Lunar Jetman: Routine at 37718

Redraw the whole strip of moon surface along the bottom of the screen for the current scroll position, then colour the row of ground beneath it.

Jetman always stands at pixel column 120 so the left-hand edge of the screen is 120 pixels behind his world X in Jetman_Position. That is divided by 8 to find the terrain cell at the left edge, as the world is 8192 pixels wide and TerrainData_Active holds one cell for every 8 of them (1024 cells), wrapping round at the far end. The 32 cells from there are drawn side by side along character row 22.

Redraw the whole strip of moon surface along the bottom of the screen for the current scroll position, then colour the row of ground beneath it. Jetman always stands at pixel column 120 so the left-hand edge of the screen is 120 pixels behind his world X in Jetman_Position. That is divided by 8 to find the terrain cell at the left edge, as the world is 8192 pixels wide and TerrainData_Active holds one cell for every 8 of them (1024 cells), wrapping round at the far end. The 32 cells from there are drawn side by side along character row 22.
Work out which terrain cell sits at the left-hand edge of the screen.
DrawTerrain	37718	LD HL,(24698)	Fetch Jetman's world X from Jetman_Position.
DrawTerrain_Target	37721	LD DE,65416	DE=65416 (minus 120).
	37724	ADD HL,DE	Subtract 120 to find the world X of the left-hand edge of the screen.
The world is 8192 pixels wide, so wrap anything left of its start back round to the far end.
	37725	LD A,H	A=H.
	37726	AND %00011111	Keep only bits 0-4 so the position wraps round the world.
	37728	LD H,A	H=A.
	37729	SRA H	Divide the world X by 8 to give the terrain cell index.
	37731	RR L
	37733	SRA H
	37735	RR L
	37737	SRA H
	37739	RR L
Start at the left-hand end of the terrain row on the screen.
	37741	LD DE,20672	Set the screen address to 20672 (the start of character row 22).
	37744	LD BC,28672	Set the terrain base in BC to TerrainData_Active.
	37747	LD A,32	Set a column counter in A for 32 loops.
Draw the graphic for one terrain cell.
DrawTerrain_Loop	37749	PUSH AF	Stash the counter and the cell index on the stack.
	37750	PUSH HL	Stash the counter and the cell index on the stack.
	37751	ADD HL,BC	Add the terrain base to get the address of the cell.
	37752	PUSH BC	Stash the terrain base and the screen address on the stack.
	37753	PUSH DE	Stash the terrain base and the screen address on the stack.
	37754	CALL DrawTerrainColumn	Call DrawTerrainColumn to copy the cell graphic onto the screen.
	37757	POP DE	Restore the screen address from the stack.
	37758	INC E	Move along to the next screen column.
	37759	POP BC	Restore the terrain base and the cell index from the stack.
	37760	POP HL	Restore the terrain base and the cell index from the stack.
Step on to the next terrain cell, wrapping round at the end of the world, and loop until every column is drawn.
	37761	INC HL	Move onto the next cell index.
	37762	LD A,H	A=H.
	37763	AND %00000011	Keep only bits 0-1 so the index wraps round after 1024 cells.
	37765	LD H,A	H=A.
	37766	POP AF	Restore the counter from the stack.
	37767	DEC A	Decrease the column counter by one.
	37768	JR NZ,DrawTerrain_Loop	Jump back to DrawTerrain_Loop until all 32 columns are drawn.
Colour the ground beneath the terrain; the black border columns at either edge of the window are left alone.
	37770	LD HL,23265	Set the attribute pointer to 23265 (the second column of the bottom row).
	37773	LD BC,7771	Set a column counter in B for 30 loops and the ground colour in C to INK:MAGENTA, PAPER:MAGENTA (BRIGHT).
DrawTerrain_WriteByte	37776	LD (HL),C	Write the ground colour to the attribute buffer.
	37777	INC HL	Move onto the next column.
	37778	DJNZ DrawTerrain_WriteByte	Decrease the column counter by one and loop back to DrawTerrain_WriteByte until the ground is coloured.
	37780	RET	Return.