format - Format a string in the style of sprintf 

 format formatString ?arg arg ...?  

This command generates a formatted string in the same way as the ANSI C sprintf procedure (it uses sprintf in its implementation). FormatString indicates how to format the result, using % conversion specifiers as in sprintf, and the additional arguments, if any, provide values to be substituted into the result. The return value from format is the formatted string.  
More at: http://tcl.activestate.com/man/tcl8.4/TclCmd/format.htm
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You can use [[format]] to produce unsigned integers for display (but don't reckon with them - for expr they're still signed!):
 % format %u -1
 4294967295
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