Memory Map and accessing offset > 255

[+mksmith]

Hi,

Been taking a little break this week from Arkanoid to recharge and wanted to test a theory I have with the code RevEng created for the brick layout...

 

I'm looking to store a map grid area in memory (say 25*25 squares) and need to (based on the x and y of character position) determine what is stored at that memory location.

dim mapTable = $2200
dim charX = a
dim charB = b
rem map area 25x25 (0-24)
const mapWidth = 24
const mapHeight = 24

rem char position
charX = 18 : charY = 22

rem get value at location
offset=charX+(charY*mapWidth) : value = mapTable[offset]

I'm getting stuck as the offset can obviously be >255. The question is what would be the most optimal way to calculate the offset in this situation? Assembly solution is acceptable.

[+mksmith]

So I've been playing around today and not getting any progress - been looking to use pointers to lookup the column (Y) and then getting the offset from that (X):

 dim pointer = a.b
 
 dim mapTable = $2200  
 dim mapTableRow0 = mapTable
 dim mapTableRow1 = mapTableRow0 + #25
 dim mapTableRow2 = mapTableRow1 + #25
 dim mapTableRow3 = mapTableRow2 + #25
 dim mapTableRow4 = mapTableRow3 + #25
 dim mapTableRow5 = mapTableRow4 + #25

 data mapTableLookupPointerLo
 <#mapTableRow0,<#mapTableRow1,<#mapTableRow2,<#mapTableRow3,<#mapTableRow4,<#mapTableRow5
end
 data mapTableLookupPointerHi
 >#mapTableRow0,>#mapTableRow1,>#mapTableRow2,>#mapTableRow3,>#mapTableRow4,>#mapTableRow5
end

 function getOffset
 rem temp1=x temp2=y
 pointer[0] = mapBlockTableLookupPointerLo[temp2] : pointer[1] = mapBlockTableLookupPointerHi[temp2]
 temp3 = pointer[[temp1]]
 return temp3

Not entirely sure if that could/should/would even work... Any suggestions on this?

[swapd0]

It must work, having an offset to each row it's a good solution.

[+SmittyB]

I've made a couple of adjustments, not sure if anything else might be needed.

 

	rem 'a.b' is used for fixed-point numbers, not sure if it will confuse other parts of 7800BASIC
	rem As long as the low and high bytes are contiguous in memory it will work
	dim pointer = a
	dim pointerHi = b
	
	dim mapTable = $2200  
	dim mapTableRow0 = mapTable
	dim mapTableRow1 = mapTableRow0 + #25
	dim mapTableRow2 = mapTableRow1 + #25
	dim mapTableRow3 = mapTableRow2 + #25
	dim mapTableRow4 = mapTableRow3 + #25
	dim mapTableRow5 = mapTableRow4 + #25

	rem Swapped around the # and >/< symbols 
	data mapTableLookupPointerLo
	#<mapTableRow0,#<mapTableRow1,#<mapTableRow2,#<mapTableRow3,#<mapTableRow4,#<mapTableRow5
end
	data mapTableLookupPointerHi
	#>mapTableRow0,#>mapTableRow1,#>mapTableRow2,#>mapTableRow3,#>mapTableRow4,#>mapTableRow5
end

	function getOffset
	rem temp1=x temp2=y
	rem Changed the names to match the data definitions
	pointer[0] = mapTableLookupPointerLo[temp2] : pointer[1] = mapTableLookupPointerHi[temp2]
	temp3 = pointer[[temp1]]
	return temp3

 

[+mksmith]

@SmittyB Hey thanks for taking a look! Hmmm... still not getting a result though...

[+SmittyB]

One of the things I wasn't sure about was how you're dimming the rows by referencing the previous location + 25. I've not tried it myself so I can't say if it works or not

[+mksmith]

No probs ? Yeah not quite sure either - might try a few variations.  Putting together a quick little test app.

[+mksmith]

@SmittyB Actually it may be working... I'll get back to you!

[+mksmith]

Ok - this does appear to work! I built a separate example to what i'm working with and it's displaying the correct values! 

 

So to set the value use the following:

pointer[0] = mapTableLookupPointerLo[Y] : pointer[1] = mapTableLookupPointerHi[Y] : pointer[[X]] = value

Thanks @SmittyB ???

[+SmittyB]

That should be fine for setting the values. Glad that it's working.