[Vm-dev] VM Maker: VMMaker-oscog-dtl.59.mcz

[Bert Freudenberg]

On 18.04.2011, at 16:46, David T. Lewis wrote:

> 
> On Mon, Apr 18, 2011 at 12:12:22PM +0200, Levente Uzonyi wrote:
>> 
>> On Mon, 18 Apr 2011, Bert Freudenberg wrote:
>> 
>>> 
>>> On 18.04.2011, at 07:23, squeak-dev-noreply at lists.squeakfoundation.org 
>>> wrote:
>>> 
>>>> Add primitiveImageFormatVersion, an optional named primitive answering 
>>>> the image format number of the current image. This is the value stored in 
>>>> the first word of an image file header when the image is saved, and 
>>>> possibly modified on image load if the VM adds or removes capabilities 
>>>> for the running image. This primitive was added to VMMaker trunk in 
>>>> VMMaker-dtl.169. Rationale: supports float word order handing for image 
>>>> segments, reference 
>>>> http://lists.squeakfoundation.org/pipermail/vm-dev/2011-April/007712.html
>>> 
>>> Is there a way to detect if an image will be saved in a different format 
>>> than it was loaded?
>>> 
>>> In some images I want to avoid saving them accidentally as a Cog image. I 
>>> have been doing this by adding a preference "warnAboutSavingAsCog" and 
>>> checking the VM version string for "cog". With this new primitive I could 
>>> more reliably detect Cog, but I guess no information about the loaded 
>>> image is retained?
>> 
>> Why don't you read the first four bytes from the image file to get the 
>> image version? Also, there's SmalltalkImage >> #isRunningCog to check if 
>> the current VM is a CogVM.
>> 
>> imageFormat := (FileStream fileNamed: Smalltalk imageName do: [ :file |
>> 	file binary; next: 4 ]) unsignedLongAt: 1 bigEndian: false.
>> isCogVM := Smalltalk isRunningCog.
> 
> The image format number in the file is not necessarily the same as the
> image format currently being used. For example, when an interpreter VM
> reads an image saved from Cog (i.e. format 6505), it converts the image
> to format 6504 prior to entering the interpreter. So the disk file will
> tell you that you are using a 6505 image, but the VM knows better and
> will report the current image format version as 6504.

Yes, that is precisely the use case here. I wanted to detect when an interpreter image was saved as cog image for the first time.

wasCog := self imageFormatVersionFromFile allMask: 1.
isCog := Smalltalk isRunningCog.

imageFormatVersionFromFile
	| format |
	format := self imageFormatVersionFromFileAsIs.
	^format <= 16r00FFFFFF
		ifTrue: [  "same endianness as VM"
			format ]
		ifFalse: [ "convert endianness"
			((format bitAnd: 16rFF000000) >> 24)
			+ ((format bitAnd: 16r00FF0000) >> 8)
			+ ((format bitAnd: 16r0000FF00) << 8)
			+ ((format bitAnd: 16r000000FF) << 24)]

imageFormatVersionFromFileAsIs
	^(FileStream fileNamed: Smalltalk imageName do: [ :file | file binary; next: 4 ])
		unsignedLongAt: 1 bigEndian: Smalltalk isBigEndian



- Bert -