C02/doc/c02.txt

INTRODUCTION

C02 is a simple C-syntax language designed to generate highly optimized
code for the 6502 microprocessor. The C02 specification is a highly
specific subset of the C standard with some modifications and extensions

PURPOSE

Why create a whole new language, particularly one with severe restrictions,
when there are already full-featured C compilers available? It can be
argued that standard C is a poor fit for processors like the 6502. The C 
was language designed to translate directly to machine language instructions 
whenever possible. This works well on 32-bit processors, but requires either 
a byte-code interpreter or the generation of complex code on a typical
8-bit processor. C02, on the other hand, has been designed to translate 
directly to 6502 machine language instructions.
 
The C02 language and compiler were designed with two goals in mind.

The first goal is the ability to target machines with low memory: a few 
kilobytes of RAM (assuming the generated object code is to be loaded into 
and ran from RAM), or as little as 128 bytes of RAM and 2 kilobytes of ROM
(assuming the object code is to be run from a ROM or PROM). 

The compiler is agnostic with regard to system calls and library functions. 
Calculations and comparisons are done with 8 bit precision. Intermediate 
results, array indexing, and function calls use the 6502 internal registers.
While this results in compiled code with virtually no overhead, it severely 
restricts the syntax of the language.

The second goal is to port the compiler to C02 code so that it may be
compiled by itself and run on any 6502 based machine with sufficient memory
and appropriate peripherals. This slightly restricts the implementation of
code structures.

SOURCE AND OUTPUT FILES

C02 source code files are denoted with the .c02 extension. The compiler 
reads the source code file, processes it, and generates an assembly 
language file with the same name as the source code file, but with
the .asm extension instead of the .c02 extension. This assembly language
file is then assembled to create the final object code file.

Note: The default implementation of the compiler creates assembly 
language code formatted for the DASM assembler. The generation of the 
assembly language is parameterized, so it may be easily changed to
work with other assemblers.

COMMENTS

The parser recognizes both C style and C++ style comments.

C style comments begin with /* and end at next */. Nested C style comments 
are not supported.

C++ style comments begin with // and end at the next newline. C++ style
comments my be nested inside C style comments.

DIRECTIVES

Directives are special instructions to the compiler. They do not directy
generate compiled code. A directive is denoted by a leading # character. 
C02 currently supports only one directive.

The #include directive causes the compiler to read and process and external
file. In most cases, #include directives will be used with libraries of
function calls, but they can also be used to modularize the code that makes
up a program.

An #include directive is followed by the file name to be included. This
file name may be surrounded with either a < and > character, or by two "
characters. In the former case, the compiler looks for the file in an
implementation specific library directory (the default being ./include), 
while in the latter case, the compiler looks for the file in the current
working directory. Two file types are currently supported.

Header files are denoted by the .h02 extension. A header file is used to
provide the compiler with the information necessary to use machine
language system and/or library routines written in assembly language,
and consists of comments and declarations. The declarations in a header 
file added to the symbol table, but do not directly generate code. After
a header file has been processed, the compiler reads and process a
assembly language file with the same name as the header file, but with
the .a02 extension instead of the .h02 extension.

The compiler does not currently generate any assembler required 
pseudo-operators, such as the specification of the target processor,
or the starting address of the assembled object code. Therefore, at least
one header file, with an accompanying assembly language file is needed
in order to successfully assemble the compiler generated code. Details
on the structure and implementation of a typical header file can be
found in the file header.txt.

Assembly language files are denoted by the .asm extension. When the
compiler processes an assembly language file, it simply inserts the contents
of the file into the generated code.

Note: Unlike standard C and C++, which use a preprocessor to process
directives, the C02 compiler processes directives directly.

CONSTANTS

A constant represents a value between 0 and 255. Values may be written as
a number (binary, decimal, osir hexadecimal) or a character literal.

A binary number consists of a % followed by eight binary digits (0 or 1).

A decimal number consists of one to three decimal digits (0 through 9).

A hexadecimal number consists of a $ followed by two hexadecimal digits 
(0 through 9 or A through F).

A character literals consists of a single character surrounded by ' symbols.
A ' character may be specified by escaping it with a \. 

Examples:
  &0101010  Binary Number
  123       Decimal Number
  $FF       Hexadecimal Number
  'A'       Character Literal
  '\''      Escaped Character Literal

STRINGS 

A string is a consecutive series of characters terminated by an ASCII null
character (a byte with the value 0).

A string literal is written as up to 255 printable characters. prefixed and 
suffixed with " characters.

SYMBOLS

A symbol consists of an alphabetic character followed by zero to five 
alphanumeric characters. Four types of symbols are supported: labels,
simple variables, variable arrays, and functions.

A label specifies a target point for a goto statement. A label is written
as a symbol suffixed by a : character.

A simple variable represents a single byte of memory. A variable is written 
as a symbol without a suffix.

A variable array represents a block of up to 256 continuous bytes in
memory. An Array reference are written as a symbol suffixed a [ character,

index, and ] character. The lowest index of an array is 0, and the highest
index is one less than the number of bytes in the array. There is no bounds
checking on arrays: referencing an element beyond the end of the array will 
access indeterminate memory locations.

A function is a subroutine that receives multiple values as arguments and 
optionally returns a value. A function is written as a symbol suffixed with
a ( character, up to three arguments separated by commas, and a ) character,

The special symbols A, X, and Y represent the 6502 registers with the same
names. Registers may only be used in specific circumstances (which are
detailed in the following text). Various C02 statements modify registers 
as they are processed, care should be taken when using them. However, when
used properly, register references can increase the efficiency of compiled
code.

STATEMENTS

Statements include declarations, assignments, stand-alone function calls,
and control structures. Most statements are suffixed with ; characters,
but some may be followed with program blocks.

BLOCKS

A program block is a series of statements surrounded by the { and }
characters. They may only be used with function definitions and control
structures. 

DECLARATIONS

A declaration statement consists of type keyword (char or void) followed 
by one or more variable names and optional definitions, or a single 
function name and optional function block. 

Variables may only be of type char and all variable declaration statements
are suffixed with a ; character.

A simple variable declaration may include an initial value definition in
the form of an = character and constant after the variable name. 

A variable array may be declares in one of two ways: the variable name
suffixed with a [ character, a constant specifying the upper bound of 
the array, and a ] character; or a variable name followed by an = character 
and string literal or series of constants separated by , characters and 
surrounded by { or } characters.

Variables are initialized at compile time. If a variable is changed during
execution, it will not be reinitialized unless the compiled program is
reloaded into memory.

Examples:
  char c;             //Defines variable c 
  char i, j;          //Defines variables i and j
  char r[7];          //Defines 8 byte array r 
  char s = "string";  //Defines 7 byte array s initialized to "string"
  char m = {1,2,3};   //Defines 3 byte array m initialized to 1, 2, and 3   

A function declaration consists of the function name suffixed with a (
character, followed zero to three comma separated simple variables and 
a ) character. A function declaration terminated with a ; character is
called a forward declaration and does not generate any code, while one 
followed by a program block creates the specified function. Functions of
type char explicitly return a value (using a return statement), while 
functions of type void do not.

Examples:
  void myfunc();            //Forward declaration of function myfunc
  char min(tmp1, tmp2) {if (tmp1 < tmp2) return tmp1; else return tmp2;}

Note: Like all variables, function parameters are global. They must be
declared prior to the function decaration, and retain there values after
the function call. Although functions may be called recursively, they are
not re-entrant. Allocation of variables and functions is implementation 
dependent, they could be placed in any part of memory and in any order.  
The default behavior is to place variables directly after the program code, 
including them as part of the generated object file. 

The return value of a function is passed through the A register. A return 
statement with an explicit expression will simply process that expression
(which leaves the result in the A register) before returning. A return
statement without an expression (including an implicit return) will, by
default, return the value of the last processed expression.

EXPRESSIONS

An expression is a sseries of one or more terms separated by operators. 

The first term in an expression may be a function call, subscripted array 
element, simple variable, constant, or register (A, X, or Y). An expression 
may be preceded with a - character, in which case the first term is assumed 
to be the constant 0.

Additional terms are limited to subscripted array elements, simple variables
and constants.

Operators:
  + — Add the following value.
  - — Subtract the following value. 
  & — Bitwise AND with the following value.
  | — Bitwise OR with the following value.
  ^ — Bitwise Exclusive OR with the following value.

Arithmetic operators have no precedence. All operations are performed in
left to right order. Expressions may not contain parenthesis. 

Note: the character ! may be substituted for | on systems that do not 
support the latter character. No escaping is necessary because a ! may
not appear anywere a | would.

After an expression has been evaluated, the A register will contain the
result. 

EVALUATIONS

An evaluation is a construct which generates either TRUE or FALSE condition.
It may be an expression, a comparison, or a test.

A stand-alone expression evaluates to TRUE if the result is non-zero, or 
FALSE if the result is zero.

A comparison consists of an expression, a comparator, and a term (subscripted
array element, simple variable, or constant).

Comparators:
  =  — Evaluates to TRUE if expression is equal to term
  <  — Evaluates to TRUE if expression is less than term
  <= — Evaluates to TRUE if expression is less than or equal to term
  >  — Evaluates to TRUE if expression is greater than term
  >= — Evaluates to TRUE if expression is greater than or equal to term
  <> — Evaluates to TRUE if expression is not equal to term

The parser considers == equivalent to a single =. The operator <>
was chosen instead of the usual != because it simplified the parser design. 

A test consists of an expression followed by a test-op.

Test-Ops:
  :+ — Evaluates to TRUE if the result of the expression is positive
  :- — Evaluates to TRUE if the result of the expression is negative
  
A negative value is one in which the high bit is a 1 (128 — 255), while a
positive value is one in which the high bit is a 0 (0 — 127). The primary
purpose of test operators is to check the results of functions that return
a positive value upon succesful completion and a negative value if an error 
was encounters. They compile into smaller code than would be generated 
using the equivalent comparison operators.

A comparison may be preceded by negation operator (a ! character), which 
reverses the meaning of the entire comparison. For example,
  ! expr
evaluates to TRUE if expr is zero, or FALSE if it is non-zero; while
  ! expr = term
evaluates to TRUE if expr and term are not equal, or FALSE if they are; and
  ! expr :+
evaluates to TRUE if expr is negative, or FALSE if it is positive

Note: Evaluations are compiled directly into 6502 conditional branch 
instructions, which precludes their use inside expressions. Standalone 
expressions and test-ops generate a single branch instruction, and 
therefore result in the most efficient code. Comparisons generate a
compare instruction and one or two branch instructions (=. <. >=, and <> 
generate one, while <= and > generate two). A preceding negation operator
will switch the number of branch instructions used in a comparison, but
otherwise does not change the size of the generated code.

ARRAY SUBSCRIPTS

Individual elements of an array are accessed using subscript notation.
Subscripted array elements may be used as a terms in an expression, as well
as the target variable in an assignments. They are written as the variable
name suffixed with a [ character, followed by an index, and the ] character.
The index may be a constant, a simple variable, or a register (A, X or Y).

Examples:
  z = r[i];  //Store the value from element i of array r into variable z
  r[0] = z;  //Store the value of variable z into the first element of r

Note: After a subscripted array reference, the 6502 X register will contain
the value of the index (unless the register Y was used as the index, in 
which X register is not changed).

FUNCTION CALLS

A function call may be used as a stand-alone statement, or as the first
term in an expression. A function call consists of the function name 
appended with a ( character, followed by zero to three arguments separated 
with commas, and a closing ) character.

The first argument of a function call may be an expression, address, or 
string (see below).

The second argument may be a term (subscripted array element, simple 
variable, or constant), address, or string,

The third argument may only be a simple variable or constant.

If the first or second argument is an address or string, then no more 
arguments may be passed.

To pass the address of a variable or array into a function, precede the
variable name with the address-of operator &. To pass a string, simply
specify the string as the argument.

Examples:
  c = getc();            //Get character from keyboard
  n = abs(b+c-d);        //Return the absolute value of result of expression
  m = min(r[i], r[j]);   //Return lesser of to array elements
  l = strlen(&s);        //Return the length of string s
  p = strchr(c, &s);     //Return position of character c in string s
  putc(getc());          //Echo typed characters to screen
  puts("Hello World");   //Write "Hello World" to screen

Note: This particular argument passing convention has been chosen because 
of the 6502's limited number of registers and stack processing instructions. 
When an address is passed, the high byte is stored in the Y register and 
the low byte in the X register. If a string is passed, it is turned into 
anonymous array, and it's address is passed in the Y and X registers. 
Otherwise, the first argument is passed in the A register, the second in 
the Y register, and the third in the X register. 

EXTENDED PARAMETER PASSING

To enable direct calling of machine language routines that that do not match
the built-in parameter passing convention, C02 supports the non-standard
statements push, pop, and inline.

The push statement is used to push arguments onto the machine stack prior
to a function call. When using a push statement, it is followed by one or
more arguments, separated by commas, and terminated with a semi-colon. An
argument may be an expression, in which case the single byte result is 
pushed onto the stack, or it may be an address or string, in which case the
address is pushed onto the stack, high byte first and low byte second.

The pop statement is likewise used to pop arguments off of the machine
stack after a function call. When using a pop statement, it is followed
with one or more simple variables, separated by commas, and terminated
with a semicolon. If any of the arguments are to be discarded, an asterisk
can be specified instead of a variable name.

The number of arguments pushed and popped may or may not be the same,
depending on how the machine language routine manipulates the stack pointer.

Examples:
  push d,r; mult(); pop p;
  push x1,y1,x2,y2; rect(); pop *,*,*,*;
  push &s, "tail"; strcat();

Note: The push and pop statements could also be used to manipulate the
stack inside or separate from a function, but this should be done with
care.
  
The inline statement is used when calling machine language routines that
expect constant byte or word values immediately following the 6502 JSR
instruction. A routine of this type will adjust the return address to the
point directly after the last instruction. When using the inline statement,
it is followed by one or more arguments, separated by commas, and 
terminated with a semicolon. The arguments may be constants, addresses,
or strings.

Examples;
  iprint(); inline "Hello World"; //Print "Hello World"
  irect(); inline 10,10,100,100;  //Draw rectangle from (10,10) to (100,100)

Note: If a string is specified in an inline statement, rather than creating
an anonymous string and compiling the address inline, the entire string will
be compiled directly inline.

ASSIGNMENTS

An assignment is a statement in which the result of an expression is stored 
in a variable. An assignment usually consists of a simple variable or 
subscripted array element, an = character, and an expression, terminated 
with a ; character.

Examples:
  i = i + 1;     //Add 1 to contents variable i
  c = getchr();  //Call function and store result in variable c
  s[i] = 0;      //Terminate string at position i
    
SHORTCUT-IFS

A shortcut-if is a special form of assignment consisting of an evaluation 
and two expressions, of which one will be assigned based on the result
of the evaluation. A shortcut-if is written as a condition surrounded 
by ( and ) characters, followed by a ? character, the expression to be 
evaluated if the condition was true, a : character, and the expression to 
be evaluated if the condition was false.

Example:
  result = (value1 < value) ? value1 : value2;
  
Note: Shortcut-ifs may only be used with assignments. This may change in
the future.  
  
POST-OPERATORS

A post-operator is a special form of assignment which modifies the value
of a variable. The post-operator is suffixed to the variable it modifies. 

Post-Operators:
  ++  Increment variable (increase it's value by 1)
  --  Decrement variable (decrease it's value by 1)
  <<  Left shift variable
  >>  Right shift variable
  
Post-operators may be used with either simple variables or subscripted
array elements.  

Examples:
  i++;    //Increment the contents variable i
  b[i]<<; //Left shift the contenta of element i of array b
  
Note: Post-operators may only be used in stand-alone statements, although
this may change in the future.

ASSIGNMENTS TO REGISTERS

Registers A, X, and Y may assigned to using the = character. Register A
(but not X or Y) may be used with the << and >> post-operators, while
registers X and Y (but not A) may be used with the ++ and -- post-operators.

IMPLICIT ASSIGNMENTS

A statement consisting of only a simple variable is treated as an 
implicit assignment of the A register to the variable in question.

This is useful on systems that use memory locations as strobe registers.

Examples:
  HMOVE;  //Move Objects (Atari VCS)
  S80VID; //Enable 80-Column Video (Apple II)

Note: An implicit assignment generates an STA opcode with the variable
as the operand.

GOTO STATEMENT

A goto statement unconditionally transfers program execution to the 
specified label. When using a goto statement, it is followed by the
label name and a terminating semicolon.

Example:
  goto end;
  
Note: A goto statement may be executed from within a loop structure
(although a break or continue statement is preferred), but should not 
normally be used to jump from inside a function to outside of it, as 
this would leave the return address on the machine stack.

IF AND ELSE STATEMENTS

The if then and else statements are used to conditionally execute blocks
of code. 

When using the if keyword, it is followed by an evaluation (surrounded by
parenthesis) and the block of code to be executed if the evaluation was true.

An else statement may directly follow an if statement (with no other 
executable code intervening). The else keyword is followed by the block
of code to be executed if the evaluation was false.

Examples:
  if (c = 27) goto end;
  if (n) q = (n/d) else putstr("Division by 0!");
  if (r[j]<r[i]) {t=r[i],r[i]=r[j],r[j]=t)}
  
Note: In order to optimize the compiled code, the if and else statements 
are to 6502 relative branch instructions. This limits the amount of 
generated code between the if statement and the end of the if/else block
to slightly less than 127 characters. This should be sufficient in most
cases, but larger code blocks can be accomodated using function calls or
goto statements.

WHILE LOOPS

The while statement is used to conditionally execute code in a loop. When
using the while keyword, it is followed by an evalution (surrounded by
parenthesis) and the the block of code to be executed while the evaluation
is true. If the evaluation is false when the while statement is entered,
the code in the block will never be executed.

Alternatively, the while keyword may be followed by a pair of empty 
parenthesis, in which case an evaluation of true is implied.

Examples:
  c = 'A' ; while (c <= 'Z') {putchr(c); c++;} //Print letters A-Z
  while() if (rdkey()) break;                  //Wait for a keypress

Note: While loops are compiled using the 6502 JMP statements, so the code
blocks may be abritrarily large.

DO WHILE LOOPS

The do statement used with to conditionally execute code in a loop at 
least once. When using the do keyword, it is followed by the block of
code to be executed, a while statement, an evaluation (surrounded
by parenthesis), and a terminating semicolon.

A while statement that follows a do loop must contain an evaluation. 
The while statement is evaluated after each iteration of the loop, and
if it is true, the code block is repeated.

Examples:
  do c = rdkey(); while (c=0);                //Wait for keypress
  do (c = getchr(); putchr(c); while (c<>13)  //Echo line to screen

Note: Unlike the other loop structures do/while statements do not use
6502 JMP instructions. This optimizes the compiled code, but limits
the amount of code inside the loop.

FOR LOOPS

The for statement allows the initialization, evaluation, and modification
of a loop condition in one place. For statements are usually used to 
execute a piece of code a specific number of times, or to iterate through
a set of values.

When using the if keyword, it is followed by a pair of parenthesis 
containing an initialization assignment statement (which is executed once), 
a semicolon separator, an evaluation (which determines if the code block 
is exectued), another semicolon separator, and an increment assignment 
(which is executed after each iteration of the code block). This is then 
followed by the block of code to be conditionally executed.

The assignments and conditional of a for loop must be populated. If an
infinite loop is desired, use a while () statement.

Examples:
  for (c='A'; c<='Z'; c++) putchr(c);       //Print letters A-Z
  for (i=strlen(s)-1;i:+;i--) putchr(s[i]); //Print string s backwards
  for (i=0;c>0;i++) {c=getchr();s[i]=c}     //Read characters into string s  

Note: For loops are compiled using the 6502 JMP statements, so the code
blocks may be abritrarily large.  A for loop generates less efficient code
more than a simple while loop, but will always execute the increment 
assignment on a continue.

BREAK AND CONTINUE

The break and continue statements are used to jump to the beginning or
end of a do, for, or while loop. Neither may be used outside of a loop.

When a break statement is encountered, program execution is transferred
to the statement immediately following the end of the block associated 
with the innermost for or while loop. When using the break keyword, it is
followed with a trailing semicolon.

When a continue statement is encountered, program execution is transferred
to the beginning of the block associated with the innermost for or while
loop. In the case of a for statement, the increment assignment is executed,
followed by the evaluation, and in the case of a while statement, the 
evaluation is executed. When using the break keyword, it is followed with 
a trailing semicolon.

Examples:
  do {c=rdkey(); if (c=0) continue; if (c=27) break;} while (c<>13);`
  for (i=0;i<strlen(s);i++) {if (s[i]=0) break; putchr(s[i]);}
  while() {c=rdkey;if (c=0) continue;putchr(c);if (c=13) break;}
  
Note: The break and continue statements may not be used inside a do/while\
loop. This may change in the future.

UNIMPLEMENTED FEATURES

The #define directive is recognized but generates an error. The exact
implementation of this directive has not yet been determined, so it has
been reserved for future use.

The #pragma directive is currently unrecognized. It may be implemented in
the future to allow the specification of assembler specific instructions.

The only type recognized by the compiler is char. Since the 6502 is an
8-bit processor, multi-byte types would generate over-complicated code. 
For this reason, pointers are not currently implemented, athough the
address of operator can be used with specific statements. In addition, 
the signed and unsigned keywords are unrecognized, due to the 6502's 
limited signed comparison functionality.

The switch and case keywords are recognized, but generate an error. There
are no plans to implement these keywords. Due to single pass nature of the
compiler, the code generated by a switch/case structure would be no more
efficient than an equivalent series of if/then/else statements.