113 lines
3.8 KiB
Plaintext
113 lines
3.8 KiB
Plaintext
Collapse OS' Forth implementation notes
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*** EXECUTION MODEL
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After having read a line through readln, we want to interpret it. As a general
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rule, we go like this:
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1. read single word from line
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2. Can we find the word in dict?
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3. If yes, execute that word, goto 1
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4. Is it a number?
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5. If yes, push that number to PS, goto 1
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6. Error: undefined word.
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*** EXECUTING A WORD
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At it's core, executing a word is pushing the wordref on PS and calling EXECUTE.
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Then, we let the word do its things. Some words are special, but most of them
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are of the compiledWord type, and that's their execution that we describe here.
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First of all, at all time during execution, the Interpreter Pointer (IP) points
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to the wordref we're executing next.
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When we execute a compiledWord, the first thing we do is push IP to the Return
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Stack (RS). Therefore, RS' top of stack will contain a wordref to execute next,
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after we EXIT.
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At the end of every compiledWord is an EXIT. This pops RS, sets IP to it, and
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continues.
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*** Stack management
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The Parameter stack (PS) is maintained by SP and the Return stack (RS) is
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maintained by IX. This allows us to generally use push and pop freely because PS
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is the most frequently used. However, this causes a problem with routine calls:
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because in Forth, the stack isn't balanced within each call, our return offset,
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when placed by a CALL, messes everything up. This is one of the reasons why we
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need stack management routines below. IX always points to RS' Top Of Stack (TOS)
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This return stack contain "Interpreter pointers", that is a pointer to the
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address of a word, as seen in a compiled list of words.
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*** Dictionary
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A dictionary entry has this structure:
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- Xb name. Arbitrary long number of character (but can't be bigger than
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input buffer, of course). not null-terminated
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- 2b prev offset
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- 1b size + IMMEDIATE flag
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- 2b code pointer
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- Parameter field (PF)
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The prev offset is the number of bytes between the prev field and the previous
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word's code pointer.
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The size + flag indicate the size of the name field, with the 7th bit being the
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IMMEDIATE flag.
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The code pointer point to "word routines". These routines expect to be called
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with IY pointing to the PF. They themselves are expected to end by jumping to
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the address at (IP). They will usually do so with "jp next".
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That's for "regular" words (words that are part of the dict chain). There are
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also "special words", for example NUMBER, LIT, FBR, that have a slightly
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different structure. They're also a pointer to an executable, but as for the
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other fields, the only one they have is the "flags" field.
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*** System variables
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There are some core variables in the core system that are referred to directly
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by their address in memory throughout the code. The place where they live is
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configurable by the RAMSTART constant in conf.fs, but their relative offset is
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not. In fact, they're mostlly referred to directly as their numerical offset
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along with a comment indicating what this offset refers to.
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This system is a bit fragile because every time we change those offsets, we
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have to be careful to adjust all system variables offsets, but thankfully,
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there aren't many system variables. Here's a list of them:
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RAMSTART INITIAL_SP
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+02 CURRENT
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+04 HERE
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+06 IP
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+08 FLAGS
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+0a PARSEPTR
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+0c CINPTR
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+0e FINDPTR
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+10 WORDBUF
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+30 SYSVNXT
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+50 RAMEND
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INITIAL_SP holds the initial Stack Pointer value so that we know where to reset
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it on ABORT
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CURRENT points to the last dict entry.
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HERE points to current write offset.
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IP is the Interpreter Pointer
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FLAGS holds global flags. Only used for prompt output control for now.
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PARSEPTR holds routine address called on (parse)
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CINPTR holds routine address called on C<
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FINDPTR holds routine address called on FIND
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WORDBUF is the buffer used by WORD
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SYSVNXT is the buffer+tracker used by (sysv)
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