10 The program stack used by programs compiled with CC65 is located in high
11 memory. The stack starts there and grows down. Arguments to functions, local
12 data etc are allocated on this stack, and deallocated when functions exit.
14 The program code and data is located in low memory. The heap is located
15 between the program code and the stack. The default size for the parameter
16 stack is 2K, you may change this for most platforms in the linker
19 Note: The size of the stack is only needed if you use the heap, or if you
20 call the stack checking routine (_stkcheck) from somewhere in your program.
22 When calling other functions, the return address goes on the normal 6502
23 stack, *not* on the parameter stack.
30 Since CC65 is a member of the Small-C family of compilers, it uses the notion
31 of a 'primary register'. In the CC65 implementation, I used the AX register
32 pair as the primary register. Just about everything interesting that the
33 library code does is done by somehow getting a value into AX, and then calling
34 some routine or other. In places where Small-C would use a secondary
35 register, top-of-stack is used, so for instance two argument function like
36 integer-multiply work by loading AX, pushing it on the stack, loading the
37 second value, and calling the internal function. The stack is popped, and the
38 result comes back in AX.
45 C functions are called by pushing their args on the stack, and JSR'ing to the
46 entry point. (See ex 1, below) If the function returns a value, it comes back
47 in AX. NOTE!!! A potentially significant difference between the CC65
48 environment and other C environments is that the CALLEE pops arguments, not
49 the CALLER. (This is done so as to generate more compact code) In normal use,
50 this doesn't cause any problems, as the normal function entry/exit conventions
51 take care of popping the right number of things off the stack, but you may
52 have to worry about it when doing things like writing hand-coded assembly
53 language routines that take variable numbers of arguments. More about that
56 Ex 1: Function call: Assuming 'i' declared int and 'c' declared
57 char, the following C code
61 in absence of a prototype generates this assembler code. I've added
64 lda _i ; get 'i', low byte
65 ldx _i+1 ; get 'i', hi byte
68 ldx #0 ; fill hi byte with 0
71 jsr _baz ; call the function
72 sta _i ; store the result
75 In presence of a prototype, the picture changes slightly, since the
76 compiler is able to do some optimizations:
78 lda _i ; get 'i', low byte
79 ldx _i+1 ; get 'i', hi byte
83 jsr _baz ; call the function
84 sta _i ; store the result
88 Note that the two words of arguments to baz were popped before it exitted.
89 The way baz could tell how much to pop was by the argument count in Y at call
90 time. Thus, even if baz had been called with 3 args instead of the 2 it was
91 expecting, that would not cause stack corruption.
93 There's another tricky part about all this, though. Note that the args to baz
94 are pushed in FORWARD order, ie the order they appear in the C statement.
95 That means that if you call a function with a different number of args than it
96 was expecting, they wont end up in the right places, ie if you call baz, as
97 above, with 3 args, it'll operate on the LAST two, not the first two.
104 CC65 does the usual trick of prepending an underbar ('_') to symbol names when
105 compiling them into assembler. Therefore if you have a C function named
106 'bar', CC65 will define and refer to it as '_bar'.
113 Supported systems at this time are: C64, C128, Plus/4, CBM 500, CBM 600/700,
114 the newer PET machines (not 2001), Atari 8bit, and the Apple ][ (thanks to
115 Kevin Ruland, who did the port).
117 C64: The program runs in a memory configuration, where only the kernal ROM
118 is enabled. The text screen is expected at the usual place ($400), so
119 50K of memory are available to the program.
121 C128: The startup code will reprogram the MMU, so that only the kernal ROM
122 is enabled. This means, there are 41K of memory available to the
125 Plus/4: Unfortunately, the Plus/4 is not able to disable only part of it's
126 ROM, it's an all or nothing approach. So, on the Plus/4, the program
127 has only 28K available (16K machines are detected and the amount of
128 free memory is reduced to 12K).
131 The C program runs in bank #0 and has a total of 48K memory available.
132 This is less than what is available on its bigger brothers (CBM
133 600/700) because the character data and video RAM is placed in the
134 execution bank (#0) to allow the use of sprites.
137 The C program runs in a separate segment and has almost full 64K of
140 PET: The startup code will adjust the upper memory limit to the installed
141 memory. However, only linear memory is used, this limits the top to
142 $8000, so on a 8032 or similar machine, 31K of memory are available to
145 APPLE2: The program starts at $800, end of RAM is $8E00, so 33.5K of memory
146 (including stack) are available.
148 Atari: The startup code will adjust the upper memory limit to the installed
149 memory, considering future graphics memory usage (which is allocated
150 at top of RAM). The programmer can specify which graphics mode is
151 about to be used by defining a variable _graphmode_used, unsigned
152 char, to the mode value (mode values like Atari DOS, 0-31).
153 (Please note that graphics mode selection isn't supported in the
154 Atari runtime lib yet!)
155 In the default case the upper memory limit will be $8035 (with Basic
156 cartridge) and $A035 (without cartridge). This is the default which
157 leaves room for the biggest possible graphics mode. If only standard
158 text mode is used (_graphmode_used = 0), the values are $9C1F (with
159 Basic) and $BC1F (no cartridge).
160 The program starts at $1F00 (to leave room for DOS), and the free
161 memory values are $6135 (24K, Basic, default mode), $8135 (32K, no
162 Basic, default mode), $7D1F (31K, Basic, mode 0) and $9D1F (39K,
164 These values are for a 48K or 64K machine.
166 Note: The above numbers do not mean that the remaining memory is unusable.
167 However, it is not linear memory and must be accessed by other, nonportable
168 methods. I'm thinking about a library extension that allows access to the
169 additional memory as a far heap, but these routines do not exist until now.
176 CC65 allows inline assembly by a special keyword named "asm". Inline assembly
177 looks like a function call. The string in parenthesis is output in the
180 Example, insert a break instruction into the code:
184 Note: The \t in the string is replaced by the tab character, as in all other
187 Beware: Be careful when inserting inline code since this may collide with
188 the work of the optimizer.
195 There are two special variables available named __AX__ and __EAX__. These
196 variables must never be declared (this gives an error), but may be used as any
197 other variable. However, accessing these variables will access the primary
198 register that is used by the compiler to evaluate expressions, return
199 functions results and pass parameters.
201 This feature is useful with inline assembly and macros. For example, a macro
202 that reads a CRTC register may be written like this:
204 #define wr(idx) (__AX__=(idx), \
205 asm("\tsta\t$2000\n\tlda\t$2000\n\tldx\t#$00"), \