Re: [Vm-dev] Byte ordering in Form (urgent)

 

Hi All,

    in trying to fix a bug in BitBLT to do with accessing a word following a bitmap I have had to understand Form and BitBlt's extensions for handling multiple byte order.  In particular, if a Form has a negative depth its byte ordering is reversed w.r.t. a Form with positive depth.

But in examining this code I think I have found an inconsistency, and I urgently need to check my understanding.  In the Form and BitBlt code, a negative depth is stated to mean a Form whose bits are in little-endian byte order:

Form methods for accessing

depth

^ depth < 0 ifTrue:[0-depth] ifFalse:[depth]

nativeDepth

"Return the 'native' depth of the receiver, e.g., including the endianess"

^depth

Form methods for testing

isBigEndian

"Return true if the receiver contains big endian pixels, meaning the left-most pixel is stored in the most significant bits of a word."

^depth > 0

isLittleEndian

"Return true if the receiver contains little endian pixels, meaning the left-most pixel is stored in the least significant bits of a word."

^depth < 0

and BitBltSimulation (the Smalltalk code for the plugin that implements BitBlt) has two instance variables destMSW and sourceMSW that are true if the destination form or source form has a positive depth:

BitBltSimulation methods for interpreter interface

loadBitBltDestForm

"Load the dest form for BitBlt. Answer false if anything is wrong, true otherwise."

... destDepth := interpreterProxy fetchInteger: FormDepthIndex ofObject: destForm.

destMSB := destDepth > 0.

destDepth < 0 ifTrue:

[destDepth := 0 - destDepth].

...

loadBitBltSourceForm

"Load the source form for BitBlt. Return false if anything is wrong, true otherwise."

...

sourceDepth := interpreterProxy fetchInteger: FormDepthIndex ofObject: sourceForm.

sourceMSB := sourceDepth > 0.

sourceDepth < 0 ifTrue:

[sourceDepth := 0 - sourceDepth].

...

But if I have a look at the actual layout of bytes in a Form I see that the meaning of depth is that if it is positive it is the *native* byte order (little endian on x86, x86-64, ARM, big end on SPARC, PowerPC):

Display fill: (0@0 extent: 1@100) rule: Form over fillColor: Color black.

(Display bits at: 1) hex '16rFF000001'

| bytes |

bytes := Display bits copy.

ByteArray adoptInstance: bytes.

bytes copyFrom: 1 to: 4 #[1 0 0 255]

i.e. the first word of the Display after writing the color black into the first 32-bit pixel is 16rFF000001 on Mac OS x86-64, or #[1 0 0 255] , which is little endian.

When the Squeak BttF VM was first written it was written on Motorola 68k Macs and PowerPC Macs, which are big-endian.  So it is natural that at the time the terms isBigEndian and isLittleEndian and destMSB sourceMSB were as they are now, because the platform was big endian.  But subsequently when the code was ported to Windows x86 by Andreas Raab, the meaning seems to me to have changed.  Now depth > 0 implies *native* byte order, not big endian byte order, and depth < 0 implies the *opposite of native* byte order, not little endian order.  Does anyone disagree?  Does anyone have corroborating or contradicting evidence?

I ask because to make sense of the code I would at least like to change the variables in BitBltSimulation to be destNativeByteOrder and sourceNativeByteOrder, and I want to either delete Form>>isBigEndian and Form>>isLittleEndian and replace it with Form>>isNativeByteOrder, or redefine isBigEndian and isLittleEndian, eg as

isBigEndian

"Return true if the receiver contains big endian pixels, meaning the left-most pixel is stored in the most significant bits of a word."

^depth > 0 == Smalltalk isBigEndian

_,,,^..^,,,_

best, Eliot