Finding Stellar 7's cos table

I really love the mame debugger. I've got a new project: to figure out how Stellar 7's code works. It's such a genius game, I want to know how it was done.

So how does it do the math?

It stores the cos table in 0xF00 to 0xFFF, where the angle goes from 0 to 255. The high bit of the entry is the sign bit and the cos value is the low 7 bits. A value of 127 represents a value of 1.0, and a value of 0 represents 0.0. 128 represents the value of -0. 255 represents a value of -1.0.

In the next installment I'll show how it does the multiplication using the table.








To display the cos table, let's do a little bit of lua to render it to the apple's screen memory. If you've got the debugger open, just single step to get the screen to update.

cpu = manager:machine().devices[":maincpu"];mem = cpu.spaces["program"]

function max(a,b) if a>b then return a else return b end end 
function min(a,b) if a<b then return a else return b end end

function clearpage(page) page = page or 1 for i=page*8192,page*8192+8192-1 do mem:write_u8(i,0) end end

function showpage(pg) pg = pg or 1 mem:write_u8(0xc057,0) mem:write_u8(0xc050,0) mem:write_u8(0xc052,0) mem:write_u8(0xc054+pg-1,0) end


function calcyaddr(line,page) page = page or 1 return line % 8 * 1024 + math.floor(line / 64) * 40 + math.floor((line%64) / 8) * 128 + (8192 * page) end

function plot(x,y,page) page=page or 1 x = max(0,x) x = min(x,279) y = max(0,y) y = min(y,191) mem:write_u8(calcyaddr(y,page)+math.floor(x/7),mem:read_u8(calcyaddr(y,page)+math.floor(x/7))|(1<<(x%7))) end


clearpage(1)
showpage(1)
for i=0,255 do cosvalue=mem:read_u8(0xf00+i) signbit=(cosvalue&0x80) cosvalue=cosvalue&0x7F if signbit~=0  then cosvalue=-cosvalue end plot(i,math.floor(96+cosvalue*(96/128))) end