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A4E0 Unpack Floating-Point Variable to FWB

Submitted by Steve Fewell

Routine:bunp
Name: Unpack Floating-Point Variable to FWB
Starting Address: &A4E0
Entry criteria: &4A and &4B (the argp) point to a 5-byte Floating-Point variable to unpack.
Exit: The FWB contains the value of the variable.

Description:

Set FWB Rounding byte = 0.
Set FWB Overflow byte = 0.
FWB Mantissa 4 = 5th byte of the packed variable
FWB Mantissa 3 = 4th byte of the packed variable
FWB Mantissa 2 = 3rd byte of the packed variable
FWB Mantissa Sign = 2nd byte of the packed variable
FWB Exponent = 1st byte of the packed variable

If the Exponent is zero, then check whether the Mantissa byte 2, Mantissa byte 3
and Mantissa byte 4 are also zero. If so, then the FWB value = 0.0 so store zero in
FWB Mantissa 1 and exit.
Mantissa 1 is formed by ORing the 2nd byte of the packed variable with #&80 to ensure that
the top bit is set - as the top bit is replaced by the sign bit in the packed form.


Disassembly for the Unpack FP Variable to FWB routine

A4E0 dA 100 065 64 41 STZ &41
A4E2   160 004 A0 04 LDY#&04
A4E4 J 177 074 B1 4A LDA (&4A),Y
A4E6 @ 133 064 85 40 STA &40
A4E8   136 88 DEY
A4E9 J 177 074 B1 4A LDA (&4A),Y
A4EB ? 133 063 85 3F STA &3F
A4ED   136 88 DEY
A4EE J 177 074 B1 4A LDA (&4A),Y
A4F0 > 133 062 85 3E STA &3E
A4F2   136 88 DEY
A4F3 J 177 074 B1 4A LDA (&4A),Y
A4F5 ; 133 059 85 3B STA &3B
A4F7   168 A8 TAY
A4F8 J 178 074 B2 4A LDA (&4A)
A4FA < 133 060 85 3C STA &3C
A4FC   208 009 D0 09 BNE 9 --> &A507
A4FE   152 98 TYA
A4FF > 005 062 05 3E ORA &3E
A501 ? 005 063 05 3F ORA &3F
A503 @ 005 064 05 40 ORA &40
A505   240 003 F0 03 BEQ 3 --> &A50A
A507   152 98 TYA
A508   009 128 09 80 ORA#&80
A50A = 133 061 85 3D STA &3D
A50C ` 096 60 RTS

 


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