Commentary - Table of Contents
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1. 1 References 1 Intel MCS 51 Microcontroller Family User s Manual Intel Inc 272383 002 edition Feb 1994 ISO ISO TR 15580 1998 Information technology Programming languages Fortran Floating point exception handling ISO 1998 4 E L Robertson Code generation and storage allocation for machines with span dependent instructions ACM Transactions on Program ming Languages and Systems 1 1 71 83 1979 Texas Instruments TMS370 and TMS370C8 8 Bit Microcontroller Family Optimizing C Compiler Users Guide Texas Instruments spnu022c edition Apr 1996 January 30 20082. a per invocation basis Commentary This is a requirement on the implementation although the as if rule might be invoked The model being described is effectively a stack based approach to the calling of functions and the handling of storage for objects they define the actual storage allocation could use a real stack or simulate one using allocated storage Storing objects in the function image or simply having a preallocated area of storage for them would pre vent a function from being called recursively having more than one call to a function in the process of being executed at the same time is a recursive invocation however the invocation occurred An implementation is required to support recursive function calls This requirement prevents implementations using a technique that was once commonly used primarily by implementations of other languages but can have different execution time semantics when recursive calls are made Ct The C Standard does not contain this requirement Other Languages Most languages require support for recursive function calls implying this requirement Common Implementations Modern processors try to separate code function image and data object definitions Accesses to the two have different characteristics which affects the design of caches for them often implemented as two separate cache areas on the chip Independently of processor support the host environment operating system may mark cert
3. an instruction that causes an interrupt to be raised after it has only partially completed is one that accesses storage if the access causes a page fault causing the instruction to be suspended while the accessed page is swapped into storage Another case is performing an access to storage using a misaligned address or an invalid address In these cases the instruction may never successfully complete External nonprocessor based interrupts are usually only processed once execution of the current instruction is complete Some processors have instructions that can take a relatively long time to execute for instance instructions that copy large numbers of bytes between two blocks of memory Depending on the design requirements on interrupt latency some processors allow these instructions to be interrupted while others do not January 30 2008 v1 1 5 2 3 Signals and interrupts object storage outside function image function call recursive function call recursive Some implementations require that functions called by a signal handler preserve information about the state of the execution environment such as register contents Developers are required to specify often by using a keyword in the declaration such as interrupt which functions must save and restore on return this information All such objects shall be maintained outside the function image the instructions that compose the executable representation of a function on
4. min min exp max min min underflow min max min max In the above table 1 0 0 0 is a shorthand notation for any non zero finite floating point number divided by a zero floating point number max is the maximum floating point number FLT_MAX DBL_MAX LDBL_MAX min is the minimum floating point number FLT_MIN DBL_MIN LDBL_MIN log and exp are mathematical library routines We believe that LIA I should be revised to match LIA 2 IEC 559 and IEEE 754 in that 1 0 0 0 should be a pole exception and 0 0 0 0 should be an undefined exception CH The C Standard specifies Clause 15 Exception handling a much richer set of functionality for dealing with exceptional behaviors While it does not go into the details contained in this C subclause they are likely of necessity to be followed by a C implementation Other Languages Some languages e g Ada Java and PL 1 define statements that can be used to control how exceptions and signals are to be handled After over 30 years floating point exception handling has finally been specified in the Fortran Standard A few languages include functionality for handling signals and interrupts but most ignore these issues 2 v1 1 January 30 2008 5 2 3 Signals and interrupts Pull Common Implementations Implementations are completely at the mercy of what signals are supported by the host environment and what interrupts are generated by the processor Gould Encore PowerNode trea
5. The New C Standard Excerpted material An Economic and Cultural Commentary Derek M Jones derek knosof co uk Copyright 2002 2008 Derek M Jones All rights reserved 5 2 3 Signals and interrupts 5 2 3 Signals and interrupts signal Rationale WGI14 N748 Commentary 270 Signals are difficult to specify in a system independent way The C89 Committee concluded that about the only thing a strictly conforming program can do in a signal handler is to assign a value to a volatile static variable which can be written uninterruptedly and promptly return A second signal for the same handler could occur before the first is processed and the Standard makes no guarantees as to what happens to the second signal A pole exception is the same as a divide by zero exception a finite non zero floating point number divided by a zero floating point number Currently various standards define the following exceptions for the indicated sample floating point operations For LIA 2 there are other operations that produce the same exceptions LIA lt Standard gt IEEE Exception LIA 1 LIA 2 IEEE 754 IEC 559 Exception undefined 0 0 0 0 sqrt 1 0 0 0 0 0 invalid 1 0 0 0 log 1 0 infinity infinity infinity infinity 0 0 infinity sqrt 1 0 pole not yet log 0 0 1 0 0 0 division by zero floating_ max max exp max max max overflow overflow max min max min max max max max underflow
6. ain areas of storage as having execute only permission Attempts to read or write to such storage from an executing program often leads to a signal being raised Applications targeted at a freestanding environment rarely involve recursive function calls Storage may also be at a premium and hardware stack support limited the Intel 8051 is limited to a 128 byte stack Some hosts allocate fixed areas in static storage for objects local to functions A call tree built at link time can be used to work out which storage areas can be shared by overlaying those objects whose lifetimes do not overlap reducing the fixed execution time memory overhead associated with such a design Many processors have span dependent load and store instructions That is a short form measured in number of bytes that can only load or store from to storage locations whose address has a small offset relative to a base address while a long form supports larger offsets When storage usage needs to be minimized it may be possible to use a short form instruction to access storage locations in the function image The usual technique used is to reserve storage for objects after an unconditional branch instruction which is accessed by the instructions close within the range supported by the short form instruction to those locations Coding Guidelines While implementations might be required to allocate objects outside of a function image developers have been known to wr
7. al argument could be used to show that an implementation did not support this requirement A nonprogrammatic approach would be to verify the requirement by understanding how the generated machine code interacted with the host processor and the characteristics of that processor This wording is not as restrictive on the implementation as it first looks The only signal that an implementation is required to support is the one caused by a call to the raise function Requiring that any developer written functions be callable from a signal handler restricts the calling conventions that may be used in such a handler to be compatible with the general conventions used by an implementation This simplifies the implementation but places a burden on time critical applications where the calling overhead may be excessive CH This implementation requirement is not specified in the C Standard 1 9p9 Other Languages Most languages don t explicitly say anything about the interruptibility of a function Common Implementations Few if any host processors allow execution of instructions to be interrupted The boundary at the completion of one instruction and starting another is where interrupts are usually responded to In the case of pipelined processors there are two commonly seen behaviors Some processors wait until the instructions currently in the pipeline have completed execution while others flush the instructions currently in the pipeline An example of
8. ite code to store values in a program image In those few cases where values are stored in this way the developers involved are very aware of what they are doing A guideline recommendation serves no purpose Example The following is one possible method that might be used to store data in a program image 272 vil January 30 2008 5 2 3 Signals and interrupts 272 include lt stdio h gt 1 2 3 extern int always_zero 0 4 static int code_ptr 5 6 void f void 8 9 No static object declarations in this function 10 11 if always_zero 1 create some dead code 12 13 14 Pad out with enough code to create storage for an int 15 A smart optimizer is the last thing we need here 16 17 always_zero 18 always_zero 19 20 21 code_ptr 22 printf This function has been called d times n code_ptr 23 24 25 void init void 26 27 28 The value 16 is the offset of the dead code from the start of the 29 function Change to suit your local instruction sizes this works 30 for gcc on an Intel x86 We also need to make sure that the 31 pointer to int is correctly aligned A reliable guess is that 32 the alignment is a multiple of the object size 33 34 code_ptr int int char f 16 amp sizeof int 1 35 code_ptr 0 36 38 int main void 39 40 init 41 for int index 0 index lt 10 index 42 fO January 30 2008 v1
9. ted both floating point and integer overflow as being the same Coding Guidelines This subclause lists those minimum characteristics of a program image needed to support signals and interrupts Such support by the implementations is only half of the story A program that makes use of signals has to organize its behavior appropriately Techniques for writing programs to handle signals or even ensuring that they are thread safe are outside the scope of these coding guidelines 271 Functions shall be implemented such that they may be interrupted at any time by a signal or may be called by a signal handler or both with no alteration to earlier but still active invocations control flow after the interruption function return values or objects with automatic storage duration Commentary This is a requirement on the implementation An implementation may provide a mechanism for the developer to switch off interrupts within time critical functions Although such usage is an extension to the standard it cannot be detected in a strictly conforming program How could an implementation s conformance to this requirement be measured A program running under an implementation that supports some form of external interrupt for instance SIGINT might be executed a large number of times the signal handler recording where the program was interrupted this would require functionality not defined in the standard Given sufficient measurements a statistic
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