Convert AT co-ordinates to PLOT co-ordinates

daveysludge

How do you convert the 2 from MC? I don't know how to convert the vertical value as they're differently ordered...

Heres what I have so far:

org 47000-2
x	defb 0
y	defb 0		;calculated plot points

	ld a,(50000-3)	;vertical AT co-ord

	ld b,22	
	sub b

	add a,a
	ADD A,A
	add a,a		;times by 8
	
	ld (x),a	;store vertical plot point
	ret

joefish

PLOT is from the bottom of the screen upward, but since it's BASIC, it's from the bottom of the 22nd character row, not the 24th. So do 175-Y to get the plot position down the screen (from 0), then divide by 8 if you want the character row.

To go the other way round, multiply the character row by 8, then do 175-Y to turn it upside down. Though that will give you the plot position of the top of the character row. For the bottom of the character row, subtract another 7 to move down 7 pixels (or simply use 168-Y instead). e.g. 21 x 8 = 168 then 175-168 = 7 or 168-168 = 0.

Multiply and divide by two by using the arithmetic shift instructions rather than ADDs.

It's not neccesarily the quickest but you can probably follow:

LD A,(nnnn)
SLA A
SLA A
SLA A
LD B,A
LD A,168
SUB B
LD (nnnn),A

Or you could reverse the order of the sum by making A
negative first, then simply adding 168:

LD A,(nnnn)
SLA A
SLA A
SLA A
NEG A
ADD A,168
LD (nnnn),A

I think this is OK. The vertical position, because it could be greater than 127, is kind of breaking the rules for negative 8-bit numbers, but since they all wrap around it should work out OK in the end.

daveysludge

Cheers mate, sorted!

Einar Saukas

It depends if you want to convert to the highest or lowest PLOT coordinate within the same character position.

Converting from AT vertical coordinate to lowest PLOT vertical coordinate (optimized):

ld a,(VERT_AT)
add a,a
add a,a
add a,a
cpl
add a,169
ld (VERT_PLOT),a

Converting from AT vertical coordinate to highest PLOT vertical coordinate (optimized):

ld a,(VERT_AT)
add a,a
add a,a
add a,a
cpl
add a,176
ld (VERT_PLOT),a

And converting from PLOT vertical coordinate to AT vertical coordinate (optimized):

ld a,(VERT_PLOT)
and $f8
rrca
rrca
rrca
cpl
add a,22
ld (VERT_AT),a