pbasic is a modern 4k basic for x86_64. that means it runs entirely within a single page, the minimum amount of physical memory alocatable to a process. all memory touched by pbasic resides in this single page including:
- the elf executable headers
- pbasic code
- pbasic data
- runtime basic data
- the stack
pbasic on linux links the following sections:
.bss.elf.text.data
.bss and .elf are 120 bytes, and overlap. following is .text and then .data
the stack pointer is set to the end of the page on startup.
most pointers stored in memory by pbasic are 16 bit. this is accomplished by using the higher 16 bits of the current page.
the macro ldaddr src, dst can do this automatically.
lines are a linked list of the form:
| linenum | next line pointer | line data |
|---|---|---|
| 16 bit unsigned line number | 16 bit compressed pointer to the next line | line bytecode |
lines are translated to the following bytecode to save space and simplify execution.
| op | format |
|---|---|
| word byte | a single byte representing a basic word (goto, print, etc.) |
| char byte | a symbol character like =, ,, ; etc. |
| token_num | token_num followed by a 4 byte number |
| token_str | token_str followed by a null-terminated ascii string |
| token_var | token_var followed by a 2 byte pointer to a 4 byte variable |
| token_var_intern | token_var_intern followed by a 2 byte pointer to the next var, then a 4 byte variable and a null-terminated string of the variable name |
| token_eof | the end of a line |
pbasic uses the 64-bit systemV calling convention for all functions, and also uses some registers to hold global values.
pbasic mostly uses 32-bit registers when possible to save a byte in instruction encodings.
| register | special purpose | saved by |
|---|---|---|
| RAX | caller | |
| RBX | reserved for statements | callee |
| RCX | arg3 | caller |
| RDX | arg2 | caller |
| RBP | base pointer | callee |
| RSI | arg1 | caller |
| RDI | arg0 | caller |
| RSP | stack pointer | callee |
| R8 | arg4 | caller |
| R9 | arg5 | caller |
| R10 | caller | |
| R11 | caller | |
| R12 | callee | |
| R13 | execution context | callee |
| R14 | callee | |
| R15 | end of heap | callee |