🚀 go-pugleaf

In article <4691@bdt.UUCP> doug@bdt.UUCP (Doug Asherman) reports a prob-
lem when he moves a value from the DL register into a byte in memory,
and then later moves this value into AX by casting the address of the
variable into a "word ptr".  He expected the value in AX to be converted
automatically from a byte to a word; was chagrined that this didn't hap-
pen; and wonders where he has encountered a bug in Borland's TASM.

Doug, you seem to be expecting the assembler to do something at a level
above what an assembler is designed to do.

You cannot "cast" your variable "__key__" from a byte to a word by pre-
fixing it with "word ptr" in your assembly program.  All this does is
force the assembler (against its better judgment) to generate a word MOV
instruction with the address of "__key__" as one of its operands.  It
does =not= have =any= effect on the data stored in the location named
"__key__", nor in the following location.

If you want the byte value in "__key__" to be sign- or zero-extended in
the AX register, you must specify this via explicit assembler code.  No
assembler will do this for you; nor, indeed, would you want it to do so
-- since, for all the assembler knows, maybe you really do want to load
both "__key__" and the variable in the following byte into AX via a
single, word-sized MOV instruction.

Your "workaround" using a CBW instruction --

		mov	al,__key__
		cbw	; sign-extend __key__

-- is actually not a workaround at all, but is a quite reasonable and
efficient way of doing what you want (given, of course, that the "sec-
onds" value will be at most 59, and will thus never have a 1 in its
high-order bit position).

Alternatives to CBW (in case "__key__" could be in the range 128-255 and
needed to be treated as unsigned) would be to do XOR AH,AH either before
or after loading AL -- or else to do XOR AX,AX before loading AL.  This
would take one more byte of code, and one more processing cycle, than a
CBW; the difference probably isn't important unless the code in question
is in the middle of a tight loop.

--
-- Rich Wales <wales@CS.UCLA.EDU> // UCLA Computer Science Department
   3531 Boelter Hall // Los Angeles, CA 90024-1596 // +1 (213) 825-5683
   "You must not drink the tea.  It is deadly to humans."