42. Stack scanner for Digital Unix on Alpha

Warning

As of 2013-05-26, the MPS is no longer supported on Digital Unix, so this document is only of historical interest.

42.1. 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.

[DEC_Assembler] describes the assembler syntax and assembler directives. It also summarises the calling conventions used. Chapters 1 and 6 were especially useful, especially chapter 6.

[DEC_Alpha_Calling_Standard] describes the calling conventions used for Digital Alpha systems. Chapter 2 was useful. But the whole document was not used as much as the previous 2 documents.

42.2. 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.

42.3. 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.

42.4. Detail Design

42.4.1. Functions

.fun.stackscan: StackScan()

.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().

42.5. References

DEC_Assembler

Digital Equipment Corporation. 1996. “Assembly Language Programmer’s Guide”.

DEC_Alpha_Calling_Standard

Digital Equipment Corporation. 1996. “Calling Standard for Alpha Systems”.