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diff --git a/04b/README.md b/04b/README.md new file mode 100644 index 0000000..f131943 --- /dev/null +++ b/04b/README.md @@ -0,0 +1,240 @@ +# stage 04 + +As usual, the source for this compiler is `in03`, an input to the [previous compiler](../03/README.md). +`in04b` contains a hello world program written in the stage 4 language. +Here is the core of the program: + +``` +main() + +function main + puts(.str_hello_world) + putc(10) ; newline + syscall(0x3c, 0) +``` + +As you can see, we can now pass arguments to functions. And let's take a look at `putc`: + +``` +function putc + argument c + local p + p = &c + syscall(1, 1, p, 1) + return +``` + +It's so simple compared to previous languages! Rather than mess around with registers, we can now +declare local (and global) variables, and use them directly. These variables will be placed on the +stack. Since arguments are also placed on the stack, +by implementing local variables we get arguments for free. There is no difference +between the `local` and `argument` keywords in this language other than spelling. +In fact, the number of agruments to a function call is not checked against +how many arguments the function has. This does make it easy to screw things up by calling a function +with the wrong number of arguments, but it also means that we can provide a variable number of arguments +to the `syscall` function. Speaking of which, if you look at the bottom of `in04b`, you'll see: + +``` +function syscall + ... + byte 0x48 + byte 0x8b + byte 0x85 + byte 0xf0 + byte 0xff + byte 0xff + byte 0xff + ... +``` + +Originally I was going to make `syscall` a built-in feature of the language, but then I realized that wasn't +necessary. +Instead, `syscall` is a function written manually in machine language. +We can take a look at its decompilation to make things clearer: + +``` +mov rax,[rbp-0x10] +mov rdi,rax +mov rax,[rbp-0x18] +mov rsi,rax +mov rax,[rbp-0x20] +mov rdx,rax +mov rax,[rbp-0x28] +mov r10,rax +mov rax,[rbp-0x30] +mov r8,rax +mov rax,[rbp-0x38] +mov r9,rax +mov rax,[rbp-0x8] +syscall +``` + +This just sets `rax`, `rdi`, `rsi`, etc. to the arguments the function was called with, +and then does a syscall. + +## functions and local variables + +In this language, function arguments are placed onto the stack from left to right +and all arguments and local variables are 8 bytes. +As a reminder, +the stack is just an area of memory which is automatically extended downwards (on x86-64, at least). +So, how do we keep track of the location of local variables in the stack? We could do something like +this: + +``` +sub rsp, 24 ; make room for 3 variables +mov [rsp], 10 ; variable1 = 10 +mov [rsp+8], 20 ; variable2 = 20 +mov [rsp+16], 30 ; variable3 = 30 +; ... +add rsp, 24 ; reset rsp +``` + +But now suppose that in the middle of the `; ...` code we want another local variable: +``` +sub rsp, 8 ; make room for another variable +``` +well, since we've changed `rsp`, `variable1` is now at `rsp+8` instead of `rsp`, +`variable2` is at `rsp+16` instead of `rsp+8`, and +`variable3` is at `rsp+24` instead of `rsp+16`. +Also, we had better make sure we increment `rsp` by `32` now instead of `24` +to put it back in the right place. +It would be annoying (but by no means impossible) to keep track of all this. +We could just declare all local variables at the start of the function, +but that makes the language more annoying to use. + +Instead, we can use the `rbp` register to keep track of what `rsp` was +at the start of the function: + +``` +; save old value of rbp +sub rsp, 8 +mov [rsp], rbp +; set rbp to initial value of rsp +mov rbp, rsp + +lea rsp, [rbp-8] ; add variable1 (this instruction sets rsp to rbp-8) +mov [rbp-8], 10 ; variable1 = 10 +lea rsp, [rbp-16] ; add variable2 +mov [rbp-16], 20 ; variable2 = 20 +lea rsp, [rbp-24] ; add variable3 +mov [rbp-24], 30 ; variable3 = 30 +; Note that variable1's address is still rbp-8; adding more variables didn't affect it. +; ... + +; restore old values of rbp and rsp +mov rsp, rbp +mov rbp, [rsp] +add rsp, 8 +``` + +This is actually the intended use of `rbp` (it *p*oints to the *b*ase of the stack frame). +Note that setting `rsp` very specifically rather than just doing `sub rsp, 8` is important: +if we skip over some code with a local variable declaration, or execute a local declaration twice, +we want `rsp` to be in the right place. +The first three and last three instructions above are called the function *prologue* and *epilogue*. +They are all the same for all functions; a prologue is generated at the start of every function, +and an epilogue is generated for every return statement. +The return value is placed in `rax`. + +## global variables + +Global variables are much simpler than local ones. The variable `:static_memory_end` in the compiler +keeps track of where to put the next global variable in memory. It is initialized at address `0x440000`, +which gives us 256KB for code (and strings). When a global variable is added, `:static_memory_end` is increased +by its size. + +## language description + +Comments begin with `;` and may be put at the end of lines +with or without code. +Blank lines are ignored. + +To make the compiler simpler, this language doesn't support fancy +expressions like `2 * (3 + 5) / 6`. There is a limited set of possible +expressions, specifically there are *terms* and *r-values*. + +But first, each program is made up of a series of statements, and +each statement is one of the following: +- `global {name}` or `global {size} {name}` - declare a global variable with the given size, or 8 bytes if none is provided. +- `local {name}` - declare a local variable +- `argument {name}` - declare a function argument. this is functionally equivalent to `local`, so it just exists for readability. +- `function {name}` - declare a function +- `:{name}` - declare a label +- `goto {label}` - jump to the specified label +- `if {term} {operator} {term} goto {label}` - +conditionally jump to the specified label. `{operator}` should be one of +`==`, `<`, `>`, `>=`, `<=`, `!=`, `[`, `]`, `[=`, `]=` +(the last four do unsigned comparisons). +- `{lvalue} = {rvalue}` - set `lvalue` to `rvalue` +- `{lvalue} += {rvalue}` - add `rvalue` to `lvalue` +- `{lvalue} -= {rvalue}` - etc. +- `{lvalue} *= {rvalue}` +- `{lvalue} /= {rvalue}` +- `{lvalue} %= {rvalue}` +- `{lvalue} &= {rvalue}` +- `{lvalue} |= {rvalue}` +- `{lvalue} ^= {rvalue}` +- `{lvalue} <= {rvalue}` - left shift `lvalue` by `rvalue` +- `{lvalue} >= {rvalue}` - right shift `lvalue` by `rvalue` +- `{function}({term}, {term}, ...)` - function call, ignoring the return value +- `return {rvalue}` +- `string {str}` - places a literal string in the code +- `byte {number}` - places a literal byte in the code + +Now let's get down into the weeds: + +A a *number* is one of: +- `{decimal number}` - e.g. `108` (note: there's no `d` prefix anymore) +- `0x{hexadecimal number}` - e.g. `0x2f` for 47 +- `'{character}` - e.g. `'a` for 97 (the character code for `a`) + +A *term* is one of: +- `{variable name}` - the value of a (local or global) variable +- `.{label name}` - the address of a label +- `{number}` + +An *lvalue* is the left-hand side of an assignment expression, +and it is one of: +- `{variable}` +- `*1{variable}` - dereference 1 byte +- `*2{variable}` - dereference 2 bytes +- `*4{variable}` - dereference 4 bytes +- `*8{variable}` - dereference 8 bytes + +An *rvalue* is an expression, which can be more complicated than a term. +rvalues are one of: +- `{term}` +- `&{variable}` - address of variable +- `*1{variable}` / `*2{variable}` / `*4{variable}` / `*8{variable}` - dereference 1, 2, 4, or 8 bytes +- `~{term}` - bitwise not +- `{function}({term}, {term}, ...)` +- `{term} + {term}` +- `{term} - {term}` +- `{term} * {term}` +- `{term} / {term}` +- `{term} % {term}` +- `{term} & {term}` +- `{term} | {term}` +- `{term} ^ {term}` +- `{term} < {term}` - left shift +- `{term} > {term}` - right shift + +That's quite a lot of stuff, and it makes for a pretty powerful +language, all things considered. To test out the language, +in addition to the hello world program, I also wrote a little +guessing game, which you can find in the file `guessing_game`. +It ended up being quite nice to write! + +## limitations + +Variables in this language do not have types. This makes it very easy to make mistakes like +treating numbers as pointers or vice versa. + +A big annoyance with this language is the lack of local label names. Due to the limited nature +of branching in this language (`if ... goto ...` stands in for `if`, `else if`, `while`, etc.), +you need to use a lot of labels, and that means their names can get quite long. But at least unlike +the 03 language, you'll get an error if you use the same label name twice! + +Overall, though, this language ended up being surprisingly powerful. With any luck, the next stage will +finally be a C compiler... |