Platform-Specific Techniques
Ophis is intended to produce cross-assembled binaries that will
run in a variety of contexts. The expectation is that most users
will be writing for emulated versions of hardware from when the
6502 chip was current, and producing files either for those
emulators or for devices that will transfer the results to real
hardware. This chapter describes the support routines and examples
to make those tasks easier.
The Commodore 64 and VIC-20
In a real sense, the Commodore 64 is the "native"
target platform for Ophis. It was the first platform targeted
and it's the one that has received the most additional
support. It's also one where the developer needs to take the
most care about exactly what kind of program they are
writing.
c64kernal.oph actually defines no
code. It merely sets up the customary names for the KERNAL
jump table routines so that you may refer to routines
like chrout and rdtim
by name.
c64header.oph is an absolutely minimal
C64 header program; it contains the one-line BASIC program
and nothing else. Smaller programs that do not require more
than four bytes of zero page do not need to do any
bankswitching or zero page caching and don't need any more
than this. The aliases provided
in c64kernal.oph may be useful, but are
not included in this header.
c64_0.oph is suitable for larger and more
sophisticated programs. It is an enhancement of the header
file developed in the previous chapter. It stores the saved
zero page values in the RAM shadowed by the KERNAL ROM, and
it also uses a different mechanism for returning to BASIC
when done that is more robust in the face of self-modifying
programs such as those produced by self-extracting
compressed executables or onefiled multipart programs. It is
used like the other header files—just include it at
the top of your source file and use RTS
to end your program—but programs that use this header
file will have all of the zero page from $02-$8F and a
contiguous chunk of program RAM from $0800-$CFFF.
libbasic64.oph is an experimental set of
macros and routines to permit the assembly programmer to
make use of the software floating point routines provided by
BASIC. It is, for obvious reasons, not compatible
with c64_0.oph, because it needs to make
use of BASIC's workspace and the ROM itself. If you wish to
use this file you should include it near the end of your
program.
vic20.oph is a header that will work for
the unexpanded VIC-20. Memory expansion
slots change where BASIC programs load, and since these
headers load in the machine language program in as the
suffix to a BASIC program, that also changes where they are
themselves loaded. There is no trickery with bankswitching
ROMs in and out—the VIC-20 does not have enough RAM to
gain anything from these techniques.
vic20x.oph does the same, but for a
VIC-20 with one or more memory expansions.
Using LIBBASIC64
The 6502's arithmetic capabilities are rather limited. To
counteract this, BASICs of the era did floating point in
software and gave BASIC programmers the full suite of
arithmetic operations. These operations are largely
unavailable to machine language programmers.
The libbasic64.oph library is an attempt to
address this. It is currently considered highly experimental,
but initial results are very promising.
BASIC stores floating point numbers in a five-byte format, but
translates them into a seven-byte format to do actual work in
two Floating Point Accumulators (FAC1 and FAC2). Ophis will
let you specify 5-byte constants with
the .cbmfloat directive, which takes a
string and produces the requisite five-byte value.
The floating point functions in BASIC all operate on FAC1 and
are relatively reliable. The
functions abs_fac1, atn_fac1, cos_fac1, exp_fac1, int_fac1, log_fac1, rnd_fac1, sgn_fac1, sin_fac1,
and tan_fac1 are all provided. Routines
that touch the FACs tend to be extremely finicky. This system
defines a set of macros and routines to manage that for you:
`f_move dest, source: Copy a five-byte floating point value from source to dest.
`fp_load src: Loads FAC1 with the floating point constant specified by src.
`fp_store dest: Saves the value of FAC1 to the named memory location.
`fp_print src: Prints out the value of FAC1 to the screen. You may want to call int_fac1 first to round it. Unlike BASIC's PRINT statement, this routine will not bracket the number with blanks.
`fp_read ptr: Attempts to convert a string to a floating point value in FAC1, in a manner similar to BASIC's VAL function.
`fp_add operand: Adds the operand to FAC1.
`fp_subtract operand: Subtracts the operand from FAC1.
`fp_multiply operand: Multiplies the operand by FAC1.
`fp_divide operand: Divides FAC1 by the operand.
`fp_pow operand: Raises FAC1 to the operand's power.
`fp_and operand: Juggles floating point-to-integer conversions to do a bitwise AND.
`fp_or operand: Likewise, but for OR.
jsr randomize: Calls RND(-TI) and leaves the (useless) result in FAC1. This seeds BASIC's random number generator with the number of clock ticks since poweron.
jsr rnd: Calls RND(1) and leaves the result in FAC1, providing a random number between 0 and 1.
jsr fac1_sign: Loads the SGN(FAC1) into the accumulator. This will be $01 if the accumulator is positive, $00 if it is zero, and $FF if it is negative. This routine is useful for branching based on the result of a floating point computation.
Other functions are available, but their preconditions are
hazier. The source file is commented with the current state of
knowledge.
To see some of these functions in action,
the examples directory includes a
program kinematics.oph, which reads numbers
in from input and computes trajectories based on them.
The Nintendo Entertainment System
The NES development community is somewhat more fragmented than
the others. A skeletal nes.oph file is
provided, but memory locations are not as consistently
named. Much sample code doesn't provide aliases for control
registers at all.
Conveniently creating runnable NES programs is somewhat
involved. Any given product was generally burned onto several
chips that were affixed to one of a large number of circuit
boards. These are often referred to as "mappers" by
developers because their effect is to implement various
bankswitching schemes. The result is a program built out of
parts, each with its own origin. A "Hello World"
sample program ships with Ophis. It does not use a bankswitcher,
but it does split its contents into a program chip and a
graphics chip, with one of two wrapper files to knit them
together into a file that other software will recognize. Samples
are given for the common iNES format and the defunct UNIF
format.
The Atari 2600 VCS
Of all the 8-bit development communities, the Atari developers
seem to be the most cohesive. The development documents
available are universal, and analysts and developers alike all
use the register names in the Stella Developer's
Guide. Ophis follows their lead, providing these
names in the header stella.oph.
The stella.oph header also replicates two
macros that appear in the header files distributed to budding
VCS developers. They are documented in the file.