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This routine computes the exponentiation of a 16 bit value. It handles only integer values. The largest result is 2^32-1 (32 bits); that makes 31 the largest possible exponent. Results larger than 2^32-1 will overflow.

The algorithm is recursive and at each iteration breaks the exponentiation in a simpler product: if the exponent is even, it will compute the exponentiation with half the exponent and square it, while if it's odd it will compute the product of the value by the value raised at the exponent minus one. The number of multiplications to be computed varies with the exponent, and the maximum is eight for the exponent 31 (31, 30, 15, 14, 7, 6, 3, 2).

The multiplication algorithm provided is tailored for this routine: it accepts 32bit values and will produce a 32bit result.

example lda #>Num1
        sta B+1
        lda #<Num1
        sta B
        lda #Num2
        jmp Exponent
; ************************************
;       Exponent
;       input:  B value to be raised
;               .A exponent
; algo:  if .A=0 res=1
;        if .A=1 res=B
;             _
;            | B if E=1
;  Exp(B,E)= | B*Exp(B,E-1) if E is odd
;            |_Exp(B,E/2)*Exp(B,E/2) if E is even
; ************************************
P = $fb
M = $62
N = $6a
;E = $2
B = $3
        beq res1        ; is E==0 ?
        lda B
        ora B+1
        beq resB        ; if B==0 or B==1 then result=B
        cmp #1
        bne ExpSub
resB    lda #0          ; E==1 | B==1 | B==0, result=B
        sta P+2
        sta P+3
        lda B
        sta P
        lda B+1
        sta P+1
res1    sta P+1         ; E=0, result=1
        sta P+2
        sta P+3
        lda #1
        sta P
ExpSub  lsr             ; E = int(E/2)
        beq resB        ; E is 1
        bcs ExpOdd      ; E is Odd
ExpEven jsr ExpSub      ; E is Even
        ldx #$3
_ldP    lda p,x         ; multiply P by itself
        sta m,x         ; P is the result of a previous mult
        sta n,x         ; copy P in M and N
        bpl _ldP
        jmp Mult32      
ExpOdd  asl             ; E = 2*int(E/2) (=E-1)
        jsr ExpSub
        ldx #$4
_ldD    lda <p-1,x      ; multiply P by B
        sta <m-1,x      ; P is the result of a previous mult
        dex             ; copy P in M
        bne _ldD
        lda B           ; copy B in N
        sta N
        lda B+1
        sta N+1
        ;lda #0
        stx N+2
        stx N+3
        jmp Mult32
Mult32          ; 32=32*32
        lda #0
        sta P
        sta P+1
        sta P+2
        sta P+3
        ldy #$20
_loop   asl p
        rol p+1
        rol p+2
        rol p+3
        asl N
        rol N+1
        rol N+2
        rol N+3
        bcc _skip
        ldx #$fc
_add    lda <p-252,x
        adc <m-252,x
        sta <p-252,x
        bne _add
_skip   dey 
        bne _loop
base/exponentiation.txt · Last modified: 2020-06-04 19:36 by verz