The modern way… using optimising C cross-compiler on Linux.

This is easier than expected, just install CC65. On any modern Linux, this is as easy as:

$ apt-get install cc65

You then get cc65 (the C compiler), ca65 (the assembler) and ld65 (the linker). They work pretty much the same as any modern toolset:

• Compile your .C source code using cc65, which will emit a .S assembly file
• Assemble your .S assembly listing using ca65, which will emit a .o binary object file
• Link your .o object files using ld65, possibly including the cc65 standard library, which will emit a .prg executable

Here's a simple makefile to get started with development. It assumes you have a ./src folder with your C source code and that you resulting binary will be placed in ./bin. Alter the paths to the location of the cc65 installation as appropriate:

# Names of the compiler and friends
CC	= cc65
AS	= ca65
LD 	= ld65

# libraries and paths
INCLUDES = --include-dir /usr/share/cc65/include
LIBS	 = --lib /usr/share/cc65/lib/pet.lib

# Compiler flags
ASM_FLAGS= --target pet
LDFLAGS	= --target pet
CFLAGS 	= --target pet -O -Oi -Or -Os

# What our application is named
TARGET	= game.prg

all: $(TARGET)

# The main application
OBJFILES = src/test.o

$(TARGET):  $(OBJFILES)
	@echo ""
	@echo "========================================"
	@echo " -= $(TARGET) =-"
	@echo ""
	@echo Linking ....
	$(LD) $(LDFLAGS) --obj $(OBJFILES) $(LIBS) -o bin/$(TARGET)
	
################################
# Main code
################################
src/test.s: src/test.c
	$(CC) $(CFLAGS) $(INCLUDES) src/test.c

src/test.o: src/test.s
	$(AS) $(ASM_FLAGS) src/test.s

###############################
#
# Clean up
#
###############################
clean:
	rm -f src/*.o
	rm -f src/*.s
	rm -f bin/$(TARGET)

• Commodore PET specific functions (which are very few) are defined in pet.h, found in the include directory of the cc65 installation.
• CC65 implementation guidance can be found here.