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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}}
+~~DISCUSSION~~

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