Spin questions

John Abshier

I don't understand part of sign-extend.· Given the following code fragement:

CON
· _clkmode = xtal1 + pll16x
· _xinfreq = 5_000_000
var
· word·· x, z
· long·· y
PUB Start
· x := -300
· y := ~~x + 50
I understand the part of sign extending to make y equal -250.· I don't understand the sing-extend being an assignment operator.· x is declared a 16 bit variable.· Where is the 32 bit sign-extended result stored? Is x a long after execution of the last line?

What does "Param|RValue|LocalValue must be globally unique, but other methods may also use the same symbol name." mean?· Does it mean that the symbol cannot be the same as a global variable, but the names can be reused from method to method?

SSteve

John Abshier said...
I don't understand part of sign-extend. Given the following code fragement:


CON

_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000

var
word x, z
long y

PUB Start

x := -300
y := ~~x + 50


I understand the part of sign extending to make y equal -250. I don't understand the sing-extend being an assignment operator. x is declared a 16 bit variable. Where is the 32 bit sign-extended result stored? Is x a long after execution of the last line?

Edit: Wrong, wrong wrong! (see Chip's post) Only when the ~~ operator is used on a line by itself is it an assignment operator. So in your example, the value of x isn't changed. The 32-bit sign-extended result is stored in a temporary location during the evaluation of the expression.

I'm not sure what the answer is to the second half of your message. It does sound self-contradictory. But that's something the compiler should catch for you, so you can probably figure it out by experimenting a little.

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OS-X: because making Unix user-friendly was easier than debugging Windows

links:
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Post Edited (SSteve) : 7/6/2006 3:04:00 PM GMT

cgracey

The sign-extend operators (~variable and ~~variable) are variable modifiers, just like ++variable·and variable--. They work on variables only and they always write back. The ~variable extends bit 7 while the ~~variable extends bit 15.


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Chip Gracey
Parallax, Inc.

SSteve

But if x is a Word, ~~ can't do anything to its value, right? (Other than making a 32-bit intermediate value for the expression.)

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OS-X: because making Unix user-friendly was easier than debugging Windows

links:
My band's website
Our album on the iTunes Music Store

Post Edited (SSteve) : 7/6/2006 3:05:33 PM GMT

SSteve

Sorry about the bad info a couple posts ago. I was thinking of the bitwise not operator, "!". It is the one that's an assignment operator when used on a line by itself.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
OS-X: because making Unix user-friendly was easier than debugging Windows

links:
My band's website
Our album on the iTunes Music Store