Volume Number: 13 (1997)
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
billKarsh@aol.com

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