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The address of this routine is found in the table of addresses. It is called indirectly via fp_calc_2, and the routine at sqr continues here.
This subroutine performs the binary operation of raising the first number, X, to the power of the second number, Y.
The subroutine treats the result X**Y as being equivalent to EXP (Y*LN X). It returns this value unless X is zero, in which case it returns 1 if Y is also zero (0**0=1), returns zero if Y is positive, and reports arithmetic overflow if Y is negative.
to_power 3851 RST $28 X, Y
3852 DEFB $01 exchange: Y, X
3853 DEFB $31 duplicate: Y, X, X
3854 DEFB $30 f_not: Y, X, (1/0)
3855 DEFB $00 jump_true to XIS0: Y, X
3856 DEFB $07
The jump is made if X=0, otherwise EXP (Y*LN X) is formed.
3857 DEFB $25 ln: Y, LN X
Giving report A if X is negative.
3858 DEFB $04 multiply: Y*LN X
3859 DEFB $38 end_calc
385A JP exp Exit via exp to form EXP (Y*LN X).
The value of X is zero so consider the three possible cases involved.
XIS0 385D DEFB $02 delete: Y
385E DEFB $31 duplicate: Y, Y
385F DEFB $30 f_not: Y, (1/0)
3860 DEFB $00 jump_true to ONE: Y
3861 DEFB $09
The jump is made if X=0 and Y=0, otherwise proceed.
3862 DEFB $A0 stk_zero: Y, 0
3863 DEFB $01 exchange: 0, Y
3864 DEFB $37 greater_0: 0, (1/0)
3865 DEFB $00 jump_true to LAST: 0
3866 DEFB $06
The jump is made if X=0 and Y is positive, otherwise proceed.
3867 DEFB $A1 stk_one: 0, 1
3868 DEFB $01 exchange: 1, 0
3869 DEFB $05 division: Exit via division as dividing by zero gives 'arithmetic overflow'.
The result is to be 1 for the operation.
ONE 386A DEFB $02 delete: -
386B DEFB $A1 stk_one: 1
Now return with the 'last value' on the stack being 0**Y.
LAST 386C DEFB $38 end_calc: (1/0)
386D RET Finished: 'last value' is 0 or 1.
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