; Calculates the 8 bit root and 9 bit remainder of a 16 bit unsigned integer in
; Numberl/Numberh. The result is always in the range 0 to 255 and is held in
; Root, the remainder is in the range 0 to 511 and is held in Reml/Remh
;
; partial results are held in templ/temph
;
; This routine is the complement to the integer square program.
;
; Destroys A, X registers.
; variables - must be in RAM
Numberl = $F0 ; number to find square root of low byte
Numberh = Numberl+1 ; number to find square root of high byte
Reml = $F2 ; remainder low byte
Remh = Reml+1 ; remainder high byte
templ = $F4 ; temp partial low byte
temph = templ+1 ; temp partial high byte
Root = $F6 ; square root
*= $8000 ; can be anywhere, ROM or RAM
SqRoot
LDA #$00 ; clear A
STA Reml ; clear remainder low byte
STA Remh ; clear remainder high byte
STA Root ; clear Root
LDX #$08 ; 8 pairs of bits to do
Loop
ASL Root ; Root = Root * 2
ASL Numberl ; shift highest bit of number ..
ROL Numberh ;
ROL Reml ; .. into remainder
ROL Remh ;
ASL Numberl ; shift highest bit of number ..
ROL Numberh ;
ROL Reml ; .. into remainder
ROL Remh ;
LDA Root ; copy Root ..
STA templ ; .. to templ
LDA #$00 ; clear byte
STA temph ; clear temp high byte
SEC ; +1
ROL templ ; temp = temp * 2 + 1
ROL temph ;
LDA Remh ; get remainder high byte
CMP temph ; comapre with partial high byte
BCC Next ; skip sub if remainder high byte smaller
BNE Subtr ; do sub if <> (must be remainder>partial !)
LDA Reml ; get remainder low byte
CMP templ ; comapre with partial low byte
BCC Next ; skip sub if remainder low byte smaller
; else remainder>=partial so subtract then
; and add 1 to root. carry is always set here
Subtr
LDA Reml ; get remainder low byte
SBC templ ; subtract partial low byte
STA Reml ; save remainder low byte
LDA Remh ; get remainder high byte
SBC temph ; subtract partial high byte
STA Remh ; save remainder high byte
INC Root ; increment Root
Next
DEX ; decrement bit pair count
BNE Loop ; loop if not all done
RTS
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