Commodore Plus 4 - Page 329

Interfacing with. BASIC 317
the exponent) to zero. The nonzero absolute value of the floating point accumula
tor can be calculated using the following formula:
[2~8*($62) + 2~16*($63) + 2"24*($64) + 2~32*($65)] * 2[($61)"128]
where ($62) means the contents of memory location $62.
Example: In this example, the USR function is defined as a routine that returns the current
contents of the specified register of the graphics chip. BASIC program is as
follows:
10 P0KE1281,0:P0KE1282,32
20 PRIFT'WHICH REGISTER IS DESIRED";:I]mjT]Sr%
30 M=USR(lT%):IPM<OTHE]SrPRI]SrT//ILLEGAL REGISTER
1TUMBER//:GOTO2O
40
Line-by-Line Explanation
10 These two POKEs change the USR vector to point to the new routine.
20 The user is asked for an integer register number.
30 The USR function performs error checking, returning a negative value if
an illegal register number was requested. An illegal request causes control
to return to line 20.
40 The contents of the register are displayed.
The machine code subroutine to process the USR function can be entered with
the machine-language monitor. This subroutine converts the register number in
the floating point accumulator to a 1-byte index, checks it for validity, and gets
the appropriate contents of the graphics chip register. The subroutine then
converts this value into a floating point number and places it in the floating point
accumulator for return. A negative number is returned to indicate an error.
Look at sign of input register number.
If negative, it is invalid, so return.
Look at the exponent of input register
number.
If zero, it is zero, so skip conversion.
Prepare to subtract.
Load accumulator with 136.
Subtract the exponent.
This is the number of times to divide by
2.
Get the mantissa.
Decrement number of times to divide.
If negative, then done, so go on.
Divide by 2.
2000
2002
2004
2006
2008
2009
200B
200D
200E
2010
2011
2013
A5
30
A5
F0
38
A9
E5
A8
A5
88
30
4A
66
3A
61
0E
88
61
62
03
LDA
BMI
LDA
BEQ
SEC
LDA
SBC
TAY
LDA
DEY
BMI
LSR
$66
$203E
$61
$2016
#$88
$61
$62
$2016