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This is a generic solution for doing labels (forward and backward
references) in an assembler program.
How to use local labels
10 # ldy,
1 $: dey,
1 $ bne,
"n $:" defines an address reference. "n $" returns the address of the
reference defined with "n $:".
How to embed local labels in your assembler
At the moment all references are forward references, meaning all
references are resolved at the end of the definition.
The Simple Resolver
The only special thing is how a label is resolved. Numref does this by
executing a resolver-word. For example, consider a two byte opcode
with the second byte as branch-offset. The resolver-word would look
: doresolve ( iaddr -- )
dup ref-addr @ - swap 1+ X c! ;
iaddr is the address of the instruction with the reference that must
be resolved. The destination address of the reference is stored at ref-addr.
The resolver-word must be registered like this:
"' doresolve TO std-resolver"
This is not a deferred word!
To support different cpu-instruction with different operand formats it
is possible to find out the type of opcode by accessing the target's
memory in doresolve. This works for very simple processors, e.g. for
6502 it is very easy to find out whether we have a 2-byte absolute
address or a 1-byte relative address.
If this method is too difficult, it is possible to store additional
information in the resolve structure.
When assembling an opcode you should find out whether the address is a
reference and then store the xt of a special resolver word in the
resolve structure by "ref-resolver !", or store some additional data
in the resolve structure by "ref-data !", if one data field is not
enough, allocate memory and use ref-data as pointer to it.
There is a heap buffer to store the references. The structure of one
1 cell ref-link
1 cell ref-flag \ mixture of tag-number
\ and tag type
1 cell ref-resolver \ xt of resolver
1 cell ref-addr \ pointer to destination or on reference
\ (start of the instruction)
1 cell ref-data \ additional information for resolver
also assembler definitions
0 value ref-marker \ tells us that address is an reference
0 value ref-now \ points to the reference we are working on
: ref-link ref-now ;
: ref-flag ref-now cell+ ;
: ref-resolver ref-now 2 cells + ;
: ref-adr ref-now 3 cells + ;
: ref-addr ref-now 3 cells + ;
: ref-data ref-now 4 cells + ;
: ref-tag-len 5 cells ;
: ref-resolve ref-resolver @ execute ;
: ref? ( -- )
false TO ref-marker ;
: forward? ( target-addr -- target-addr false | true )
dup there = ref? and dup
IF nip THEN ;
:noname false TO ref-marker ; propper8 chained
variable ref-heap 0 ref-heap !
' drop value std-resolver
: ref! ( flags/nr -- )
\G stores a reference tag
\ get mem for tag
ref-tag-len allocate throw to ref-now
\ build link
ref-heap @ ref-link ! ref-link ref-heap !
there ref-adr !
std-resolver ref-resolver !
ref-flag ! ;
: $ ( num -- address )
\G makes a reference source with the next instruction
01ff and 0200 or ref! there ;
: $: ( num -- )
\G makes a reference target
01ff and 0a00 or ref! ;
: g$: ( num -- )
\G makes a reference target for a global label
01ff and 0e00 or ref! ;
: g$ ( num -- addr )
\G searches a global label and gets its address
01ff and 0e00 or
ref-heap BEGIN dup >r @ dup WHILE 2dup cell+ @ =
IF nip to ref-now
ref-link @ r> !
ref-now free throw EXIT THEN
REPEAT 2drop -1 ABORT" could not resolve G label!" ;
: kill$: ( -- )
\G deallocs the complete reference heap
ref-heap @ BEGIN dup WHILE dup @ swap free throw REPEAT drop
0 ref-heap ! ;
: find$: ( adr nr -- )
BEGIN dup >r @ dup WHILE 2dup cell+ @ =
IF nip to ref-now
REPEAT 2drop -1 ABORT" could not resolve label!" ;
ref-heap dup >r @
BEGIN dup WHILE dup cell+ @ 0E00 and 0200 =
IF to ref-now
ref-link @ r@ !
ref-adr @ ref-flag @ ( 01ff and ) find$:
r> to ref-now
ref-link ( dup >r ) @
ref-now free throw
dup >r @
REPEAT r> drop drop kill$: ;
' solve$ end-code8 chained