Commodore Plus 4 - Page 328

316 Machine Language on the Commodore Plus/4
SFFF3 IOBASE
Registers Altered: .X, .Y
Returns $00 (the low byte of the 1/ O base address) in .X and $FD (the high byte
of the I/O base address) in .Y. This subroutine is useful only for programs
intended to run on other Commodore computers.
$FFF6 RESET
Registers Altered: all
Does a warm reset of the Plus/4. If the STOP key is detected, this routine goes
to the MONITOR, leaving the contents of memory intact. If STOP is not
detected, the Plus/4 goes to BASIC with the contents of memory reinitialized.
Interfacing with. BASIC
Sometimes it is desirable to combine BASIC and machine language to get the
advantages of both. This can be done in a number of ways, including changing the
operating system vectors ($0300-$0331). The most common methods include the
USR function, the SYS command, and adding a BASIC wedge (a custom
command set). These methods are discussed in this section.
The USR Function
USR is a floating point function. The syntax is
TJSR(x)
where * is a numeric expression. The USR function places the value of its
argument in floating point accumulator #1 ($61-$66) and then passes control of
processing to the address contained in the USR vector at $0501-$0502
(1281-1282). These locations are initialized so as to cause an ILLEGAL QUAN
TITY error if the USR function is called without changing their values. The USR
function returns to the BASIC program the value in floating point accumulator
#1 when its execution is complete. The machine code routine pointed to by the
USR vector must be terminated with an RTS to return control to BASIC.
The Floating Point Accumulator Locations $61-$66 comprise the value con
tained in floating point accumulator #1. The number is stored in exponential
form. The absolute value resides in $61-$65. Bit 7 (the sign bit) of location $66
indicates the sign of the number (if bit 7 is set to 1, the number is negative;
otherwise it is nonnegative). A value of zero is indicated by setting $61 (which is