STACK SCANNER FOR DIGITAL UNIX / ALPHA SYSTEMS
                           design.mps.sso1al
                               draft doc
                             drj 1997-03-27

INTRODUCTION

.readership: Any MPS developer.

.intro: This is the design for Stack Scanner module that runs on DIGITAL UNIX / 
Alpha systems (See os.o1 and arch.al).  The design adheres to the general 
design and interface described (probably not described actually) in 
design.mps.ss.

.source.alpha: book.digital96 (Alpha Architecture Handbook) describes the Alpha 
Architecture independently of any particular implementation.  The instruction 
mnemonics and the semantics for each instruction are specified in that document.
.source.as: 
<URL:http://www.partner.digital.com/www-swdev/pages/Home/TECH/documents/Digital_
UNIX/V4.0/AA-PS31D-TET1_html/TITLE.html> (Assembly Language Programmer's Guide) 
describes the assembler syntax and assembler directives.  It also summarises 
the calling conventions used.  Chapters 1 and 6 were especially useful, 
especially chapter 6.
.source.convention: 
<URL:http://www.partner.digital.com/www-swdev/pages/Home/TECH/documents/Digital_
UNIX/V4.0/AA-PY8AC-TET1_html/TITLE.html> (Calling Standard for Alpha Systems) 
describes the calling convetions used for Digital Alpha systems.  Chapter 2 was 
useful.  But the whole document was not used as much as the previous 2 
documents.


DEFINITIONS

.def.saved: Saved Register.  A saved register is one whose value is defined to 
be preserved across a procedure call according to the Calling Standard.  They 
are $9-$15, $26, and $30.  $30 is the stack pointer.
.def.non-saved: Non-Saved Register.  A non-save register is a register that is 
assumed to be modified across a procedure call according to the Calling 
Standard.
.def.tos: Top of Stack.  The top of stack is the youngest portion of the stack.
.def.bos: Bottom of Stack.  The bottom of stack is the oldest portion of the 
stack.
.def.base: Base.  Of a range of addresses, the base is the lowest address in 
the range.
.def.limit: Limit.  Of a range of addresses, the limit is "one past" the 
highest address in the range.


OVERVIEW

.overview: The registers and the stack need to be scanned.  This is achieved by 
storing the contents of the registers into a frame at the top of the stack and 
then passing the base and limit of the stack region, including the newly 
created frame, to the function TraceScanAreaTagged.  TraceScanAreaTagged 
performs the actual scanning and fixing.


DETAIL DESIGN

Functions

.fun.stackscan:

Res StackScan(ScanState ss, Addr *StackBot);

.fun.stackscan.asm: The function is written in assembler.  
.fun.stackscan.asm.justify: This is because the machine registers need to be 
examined, and it is only possible to access the machine registers using 
assembler.
.fun.stackscan.entry: On entry to this procedure all the non-saved (temporary) 
registers that contain live pointers must have been saved in some root (usually 
the stack) by the mutator (otherwise it would lose the values).  Therefore only 
the saved registers need to be stored by this procedure.
.fun.stackscan.assume.saved: We assume that all the saved registers are roots.  
This is conservative since some of the saved registers might not be used.
.fun.stackscan.frame: A frame is be created on the top of the stack.  
.fun.stackscan.frame.justify: This frame is used to store the saved registers 
into so that they can be scanned.
.fun.stackscan.save: All the saved registers, apart from $30 the stack pointer, 
are to be stored in the frame.  .fun.stackscan.save.justify: This is so that 
they can be scanned.  The stack pointer itself is not scanned as the stack is 
assumed to be a root (and therefore a priori alive).
.fun.stackscan.call: TraceScanAreaTagged is called with the current stack 
pointer as the base and the (passed in) StackBot as the limit of the region to 
be scanned.   .fun.stackscan.call.justify: This function does the actual 
scanning.  The Stack on Alpha systems grows down so the stack pointer (which 
points to the top of the stack) is lower in memory than the bottom of the stack.
.fun.stackscan.return: The return value from TraceScanAreaTagged is used as the 
return value for StackScan.