The BBC and Master Computer Public Domain Library

A26C (?) / A279 Multiply FWA Mantissa by 10

Submitted by Steve Fewell

Routine:FWAMantissaMult10
Name: Floating-Point Division (FWA=FWB/FWA)
Starting Address: &A279
Entry criteria: The FWA contains the number to multiply by 10.
Exit: FWA Mantissa has been multiplied by 10.

Description:
A26C (Not used by any routines so far described - context not fully understood)

A = FWA Mantissa byte 1. Divide by 8 [move top 4 bits to bottom 4 bits]
gosub A2CE :- Convert A to ASCII & put ASCII value back at the end of string (&0600)
A = ?&F0.
Test & reset bits in &31 :- Set top 4 bits of &31 to zero (to reduce the value by 48)
thus converting the digit from ASCII to decimal.

A279 : Multiply the FWA's Mantissa by 10.
Temporarily backup A on the stack.
X = ?&34 - FWA Mantissa bit 4
Push ?&31 [FWA Man 1] to Stack
Push ?&32 [FWA Man 2] to Stack
Push ?&33 [FWA Man 3] to Stack

A = ?&35 [FWA Rounding]
A = A * 2 (Transferring the overflow & effect of the multiplication through to
&34, &33, &32, &31 (the rest of the mantissa).
A = A * 2 again (Transferring the overflow & effect of the multiplication through to
&34, &33, &32, &31 (the rest of the mantissa).

Add ?&35, store result in &35.
Add original ?&34 [stored in X] to &34
Add original ?&33 [stored on stack] to &33
Add original ?&32 [stored on stack] to &32
Add original ?&31 [stored on stack] to &31
Multiply &35, &34, &33, &32, &31 (the FWA mantissa) by 2

This in effect does Mantissa = ((Mantissa * 2) + Mantissa) * 2
Which is: Mantissa = (Mantissa * 5) * 2 or Mantissa = Mantissa * 10.

Disassembly for the FWA Mantissa multiplied by 10 routine

A26C	1	165 049	A5 31	LDA &31
A26E	J	074	4A	LSR A
A26F	J	074	4A	LSR A
A270	J	074	4A	LSR A
A271	J	074	4A	LSR A
A272		032 206 162	20 CE A2	JSR &A2CE
A275		169 240	A9 F0	LDA#&F0
A277	1	020 049	14 31	TRB &31
A279	H	072	48	PHA
A27A	4	166 052	A6 34	LDX &34
A27C	1	165 049	A5 31	LDA &31
A27E	H	072	48	PHA
A27F	2	165 050	A5 32	LDA &32
A281	H	072	48	PHA
A282	3	165 051	A5 33	LDA &33
A284	H	072	48	PHA
A285	5	165 053	A5 35	LDA &35
A287		010	0A	ASL A
A288	&4	038 052	26 34	ROL &34
A28A	&3	038 051	26 33	ROL &33
A28C	&2	038 050	26 32	ROL &32
A28E	&1	038 049	26 31	ROL &31
A290		010	0A	ASL A
A291	&4	038 052	26 34	ROL &34
A293	&3	038 051	26 33	ROL &33
A295	&2	038 050	26 32	ROL &32
A297	&1	038 049	26 31	ROL &31
A299	e5	101 053	65 35	ADC &35
A29B	5	133 053	85 35	STA &35
A29D		138	8A	TXA
A29E	e4	101 052	65 34	ADC &34
A2A0	4	133 052	85 34	STA &34
A2A2	h	104	68	PLA
A2A3	e3	101 051	65 33	ADC &33
A2A5	3	133 051	85 33	STA &33
A2A7	h	104	68	PLA
A2A8	e2	101 050	65 32	ADC &32
A2AA	2	133 050	85 32	STA &32
A2AC	h	104	68	PLA
A2AD	e1	101 049	65 31	ADC &31
A2AF	5	006 053	06 35	ASL &35
A2B1	&4	038 052	26 34	ROL &34
A2B3	&3	038 051	26 33	ROL &33
A2B5	&2	038 050	26 32	ROL &32
A2B7	*	042	2A	ROL A
A2B8	1	133 049	85 31	STA &31
A2BA	h	104	68	PLA
A2BB	`	096	60	RTS