Retargetting the library

Variant selection

fp_type

--------------------------------------------------------
=linked   |causes the object module containing the      
          |floating point emulator to be included in the
          |library (and linked into any image), along   
          |with a small interface module to take control
          |of the illegal instruction vector on startup,
          |and relinquish it on closedown.              
--------------------------------------------------------
=module   |floating point emulation is provided         
          |externally in some way (present in ROM, for  
          |example). In this case, if the               
          |target-dependent kernel follows the code of  
          |the riscos example, functions                
          |__fp_initialise, __fp_finalise and           
          |__fp_address_in_module must be provided.     
--------------------------------------------------------

memcpy

--------------------------------------------------------
=small    |memcpy, memmove and memset are implemented by
          |generic C code (which attempts to do as much 
          |as possible in word units): each occupies    
          |about 100 bytes.                             
--------------------------------------------------------
=fast     |memmove and memcpy are implemented together  
          |in assembler, which attempts to do the bulk  
          |of the move 8 words at a time using LDM/STM  
          |(about 1200 bytes). memset is implemented    
          |similarly (about 200 bytes).                 
--------------------------------------------------------

divide

--------------------------------------------------------
=small     |the fully rolled implementations.           
--------------------------------------------------------
=unrolled  |unsigned and signed divide are unrolled 8   
           |times for greater speed, but obviously use  
           |more code. Complete unrolling of divide is  
           |possible, but should be done with care since
           |the significant size increase might give    
           |decreased rather than increased performance 
           |on a cached ARM. Whichever variant is       
           |selected, fast unsigned and signed divide by
           |10 are included.                            
--------------------------------------------------------

stack=

stdfile_redirection

--------------------------------------------------------
=on  |_main extracts Unix-style stdstream connection    
     |directives from the image's argument string (<    
     |, >, >>, >&, 1>&2).           
--------------------------------------------------------

backtrace

--------------------------------------------------------
=on  |the default signal handler ends by producing a    
     |call-stack traceback to stderr. (Use of this      
     |variant is not encouraged, since it increases the 
     |proportion of the library that is linked into all 
     |images, while providing functionality better      
     |obtained from a separate debugger).               
--------------------------------------------------------

Basic choices

Address space model

A third variant, like the first, but with the stack outside of the heap and not under the application's control, can easily be synthesised. This may be a more appropriate variant if there is a skeletal operating system which implements an address-mapped stack segment.

I/O model