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FADD subroutine (address $F46E)
Purpose: To add two numbers in floating point form.
Entry: The two addends are in FP1 and FP2 respectively. For maximum
precision, both should be normalized.
Uses: SWPALGN, ADD, NORM, RTLOG.
Exit: The normalized sum is left in FP1. FP2 contains the addend of
greatest magnitude. E is altered but sign is not. The A-REG is altered
and the X-REG is cleared. The sum mantissa is truncated to 24 bits.
Caution: Overflow may result if the sum is less that -2^128 or greater than
+2^128-1. If so, a jump to location $3F5 is executed leaving 0 in X1, and
twice the proper sum in the mantissa M1. The sign bit is left in the
carry, 0 for positive, 1 for negative.
_____ __________
| | | |
FP1: | 0 | | X.YYY... |
|_____| |__________|
X1 M1
(For carry=0, true sum=+X.YYY x 2^128)
Example: Prior to calling FADD, FP1 contains +12 and FP2 contains -5.
_____ _____ _____ _____
| | | | | | | |
FP1: | $83 | | $60 | | 0 | | 0 | (+12)
|_____| |_____| |_____| |_____|
X1 M1
_____ _____ _____ _____
| | | | | | | |
FP2: | $82 | | $B0 | | 0 | | 0 | (-5)
|_____| |_____| |_____| |_____|
X2 M2
After calling FADD, FP1 contains +7 (FP2 contains +12).
_____ _____ _____ _____
| | | | | | | |
FP1 | $82 | | $70 | | 0 | | 0 | (+7)
|_____| |_____| |_____| |_____|
X1 M1