Feb 97 Tips
Volume Number: 13
Issue Number: 2
Column Tag: Tips & Tidbits
Tips & Tidbits
By Steve Sisak
I hope this surprises at least some readers and helps avoid a bug or two. Here's a
self-test to see what you know about subtraction. Be honest.
On a PPC chip, which of the following sets the carry bit (assuming an instruction form
that sets the carry is used)?
a) 0 - 0
b) 0 - 1
On a 68K chip, which sets the carry?
c) 0 - 0
d) 0 - 1
Did you answer (a) and (d)? That's right. Subraction is defined differently on PPC
than on 68K, which might catch you off guard, especially if you're an old hand at 68K
asm.
On 68K, sub d0,d1 is defined as d1 = d1 - d0, and the carry behaves like a simple
borrow. However, PPC makes direct use of the definition -n = NOT(n) + 1 and, subf
r5,r3,r4 is defined as r5 = r4 + NOT(r3) + 1, making the XER[CA] bit (for
instructions that update it) reflect an addition carry instead of a borrow.
This difference propagates to extended subtraction. You would do extended subtraction
similarly on either chip.
68K:
1) sub low words.
2) subx high words.
PPC:
1) subfc low words.
2) subfe high words.
However, the internal details are different.
subx d0,d1 --> d1 = d1 - d0 - X bit.
subfe r5,r3,r4 --> r5 = r4 + NOT(r3) + XER[CA].
There are many other examples where the carry works completely differently on the
two chips (e.g., arithmetic right-shifts).
68K: carry is set (for any type of source) if and only if the LAST bit shifted out of the
right was a one.
PPC: carry is set only if the source was negative and ANY ones are shifted out of the
right.
Yes, it's all in the manuals, but who reads manuals?
Bill Karsh
1) When patching a libraray, _Open needs more param block fields cleared than does
_OpenDF.
2) when changing the font of a (popup) menu by hooking the MDEF, LastSPExtra must
be set to -1 to clear the font cache both before calling the MDEF and after restoring the
font stuff.
Tony Nelson
tonyn@tiac.net
Having trouble getting Apple Menu Options' hierarchical menus to show? You may not
have called MaxApplZone() early in your application. The various patches AMO
installs require sufficient heap to draw the menus; if you don't expand your heap to its
maximum early on, the patches may not have the right amount of memory available
when first called, and your hierarchical Apple menu will not draw.
Brian Bechtel
blob@apple.com
Apple's System 7.5.3 (aka System 7.5 Update 2.0) adds a slick new way to quickly get
around while in a StandardFile dialog. While the dialog is up, click on the desktop or
any folder open on your desktop. You did not receive the annoying beep you expected as
in 7.5.1 and earlier - instead, StandardFile took you right to that directory! This is
invaluable when you have your deeply-nested work folder opened on the desktop and
StandardFile decides to stick you in some remote location. Now anything visible is one
click away! If Apple would give us the new movable modal file dialogs, we could really
take advantage of this. These are coming in Copland, but I am hoping they will appear
in System 7.6 that has OpenDoc built-in.
Bill Hayden
nikol@codewell.com