diff options
Diffstat (limited to 'interpret.e')
| -rw-r--r-- | interpret.e | 76 |
1 files changed, 68 insertions, 8 deletions
diff --git a/interpret.e b/interpret.e index 2bb9e43..235c98b 100644 --- a/interpret.e +++ b/interpret.e @@ -50,23 +50,83 @@ ." name of latest: " latest @ entry-to-name emitstring newline newline ; -~ TODO this is identical to the flatassembler version, but it needs to fix the -~ conflict with s" +~ Allocate space by incrementing "here", and output a word entry header in +~ it. Also add it to the "latest" linked list. Use zero as the flag values; +~ accept a string pointer on the stack and use its contents as the name. +~ +~ This is the first step of creating a new word. Its responsibility includes +~ everything up to the codeword, not including the codeword; it leaves things +~ all set up to start appending contents to the new word by calling ",". +~ +~ There's a handy diagram of the entry header format under "quick +~ reference", in the description of the exeuction model in evoke.e. Create is +~ responsible for everything up to the codeword, not including it. +~ +~ When a word is created in interpret mode using s" to provide a string +~ literal, the temporary space that s" uses is in the same place as the +~ entry header we're going to write out. It really is very useful to have +~ that work. Fortunately, it does! We're able to avoid needing a special case +~ by doing things in a very careful way, as described below. +~ +~ (string pointer --) : create + ~ We add one to the string length in order to include the trailing null + ~ terminator. This will be the length of our name field; we save an extra + ~ copy of it to help with packing later. + dup stringlen 1 + dup 3unroll + ~ (name field length, string pointer, name field length) + + ~ We use memmove to put the string in its final position, because it works + ~ correctly when the destination overlaps with the source. Notice that we + ~ do this before writing anything else in the entry header, to avoid + ~ stepping on it. The name string always starts ten bytes into the header, + ~ so we can use a fixed offset. + here @ 10 + 3unroll memmove + ~ (name field length) + + ~ Now we can get back to the fields that belong at the start of the entry + ~ header. We take the value of "here" and keep a working copy of it on the + ~ stack, which we'll advance every time we write more bytes. here @ + ~ (name field length, updated "here" pointer) + + ~ Pack the old value of "latest" as the first field of the header, linking + ~ from the newly-defined word to the next-newest word. + ~ + ~ All the entries form a linked list, from newest to oldest. Since the + ~ link is the first field in the entry header, you can get from each entry + ~ to the one before it just by dereferencing the entry pointer. latest @ pack64 + + ~ This is the flags byte. It starts at zero; our caller can change it if + ~ desired. 0 pack8 + + ~ This is the "other" null terminator, used when traversing the name + ~ string backwards for execution-token-to-entry. Yes, the name is + ~ null-terminated at both ends. 0 pack8 - swap packstring + + + ~ The name field is already populated, so just skip past it. + ~ (updated "here" pointer) + + ~ The codeword is aligned to a machine-word boundary, and the padding for + ~ it is create's responsibility. + ~ + ~ By adding the null terminator before adding alignment padding, we've + ~ made sure there's always at least one null byte. Otherwise we'd be missing + ~ the terminator if by chance the name were exactly the wrong length. 8 packalign + ~ (updated "here" pointer) + + ~ Retrieve the value of "here", which still doesn't reflect our additions, + ~ and store it at the adddress of "latest". It's the start of our + ~ newly-defined word, which makes it the latest word. here @ latest ! - here ! ; -latest @ describe -s" foo" create -latest @ describe + ~ Finally, we write our updated value of "here" back into the variable. + here ! ; -~ create 0000001000017fa8 ~ , 0000001000018080 ~ self-codeword 00000010000180d0 ~ variable 0000001000018128 |