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====== lDebug expression evaluator learns to use types (poorly) ======

One week ago I finally had to confront
a long-standing defect in lDebug's
expression evaluator: It didn't support
types. I found out because all numbers
that were "negative" in two's complement
(ie values that are >= ''8000_0000h'' unsigned)
were accepted for narrower callers,
that is, ''getword'' or ''getbyte''.
These functions are called for things
such as offsets, E/S/F command strings,
and permanent breakpoint counters.

===== =====

I didn't notice for a long time
because I only used fitting values
or -1, the latter which happened to
be allowed through because of a bug.
The bug was that the ''getexpression'' [[https://hg.pushbx.org/ecm/ldebug/file/release4/source/expr.asm#l3710|counting of significant bits]]
ended up with a count of 32 bits for
large numbers, yet [[https://hg.pushbx.org/ecm/ldebug/file/release4/source/expr.asm#l2973|the protocol defined]]
the least **five** bits
as holding the count.
Therefore a count of 32 overflowed to 0 and
the callers that did check the count
(only [[https://hg.pushbx.org/ecm/ldebug/file/release4/source/expr.asm#l3898|getword and getbyte]]
at the time) never detected that these
large numbers were outside their limits.

Now [[https://hg.pushbx.org/ecm/ldebug/rev/4ed8538a3839|the obvious fix]]
was to use six bits to return the count.
But that broke the acceptance of negative
values such as -1. So
[[https://hg.pushbx.org/ecm/ldebug/rev/7daeab2a6f55|the next step]]
was to treat all numbers as signed.
As marked in the changeset description
we considered this a magic hack though.

[[https://hg.pushbx.org/ecm/ldebug/rev/240c298eb590|The proper fix]]
came along briefly after the magic:
Almost complete support for types.
In particular, the unary minus operator
sets its value's type to negative,
allowing literals prefixed by a
unary minus to be handled as signed
numbers by the significant bits counting.
Explicit type keywords such as ''sword''
(signed word) versus ''word'' (unsigned word)
also allow to specify signedness.
Other than that, binary operators
have a complicated relationship to types.
As noted in the changeset message
we may revisit some of those later on.

Eventually [[https://hg.pushbx.org/ecm/ldebug/rev/30835aa820b2|one test]]
had to be dropped because it passed
a literal of ''FFFF_FFFF'' for a
permanent breakpoint's counter.
(Literals without any type keyword or
operator are considered unsigned
by default, like most all expression terms,
except for parens-surrounded subexpressions.)
This is now considered broken and
unsupported. Note that minus one is
still allowed, as desired.

The type support soon was used to
[[https://hg.pushbx.org/ecm/ldebug/rev/94a9fc84c2ea|implement a feature]]
we'd long desired:
specifying a ''pointer'' type
allows to use a 32-bit expression
as a 16:16 far pointer.
This is most useful with compound
variables like ''csip'' and ''sssp''
or with a memory dword that we want to
read as a far pointer without specifying
the memory address twice, eg ''g pointer [ss:sp]''
to return from a 16-bit far function
or interrupt handler. (This previously
required a command like
''g word [ss:sp + 2]:word [ss:sp]''.)
Besides, ''getaddr'' now also re-uses
the check for 16 significant bits.

Another change that came along
was checking the significant bit widths
for [[https://hg.pushbx.org/ecm/ldebug/rev/fdfcb7b7569d|R commands]]
that write to a variable or memory location.
The need for this check had been noted
[[https://hg.pushbx.org/ecm/ldebug/rev/7fd3b6e6d8ba#l2.104|in a comment]]
that originated in an early revision, in 2010 October.
This had the knock-on effect of failing a
test that used a command
reading ''r ax ^:= -1''
to bitwise invert a 16-bit register.
We [[https://hg.pushbx.org/ecm/ldebug/rev/d20838edbe71|fixed this]]
by inserting a ''word'' type keyword.

Writing of old revisions, the lack of
types in the expression evaluator
goes back all the way to
[[https://hg.pushbx.org/ecm/ldebug/rev/5eef5847c863#l14.7777|the initial commit]]
of lDebug in 2010 (still called
NDebug at the time).


{{tag>ldebug expr bugfix types}}


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