On Mon, Jan 16, 2012 at 3:08 PM, Charles Forsyth <[email protected]>wrote:
> > You should read /sys/doc/asm.pdf first. > careful: TOS is only for 68k. nothing else defines or uses it. > > Plan 9 doesn't use a base pointer, because everything can be addressed > relative to the stack pointer, > and the loader keeps track of the SP level. thus FP is a virtual register, > that the loader implements > by replacing offsets relative to it by the current appropriate offset from > the hardware stack pointer register (whatever > that might be on a given platform). That's esp on the x86. the TEXT > directive specifies the space a function > requires for its stack frame, and the loader then adds appropriate code at > start and end to provide it. > 0(FP) is the first argument, 4(FP) is the second, and so on. 0(SP) is the > bottom of the current frame, > and 0(SP), 4(SP) etc are referenced to build the arguments for outgoing > calls (but that space must > be accounted for in the TEXT directive). > > (it's probably not very different in effect from -fno-frame-pointer or > whatever it is for gcc, > which also doesn't use ebp except that is implemented entirely by the > compiler.) > > On 16 January 2012 12:30, Alexander Kapshuk > <[email protected]>wrote: > >> i have a question about putting things on the stack for x86 arch under >> plan 9... >> >> under unix/linux, when defining a function, i would: >> (1). push the address the base pointer is pointing to prior to this >> function being called, onto the stack; e.g. pushl %ebp >> (2). then i would have the base pointer point to the current stack >> pointer; e.g. movl %esp, %ebp >> (3). then i would allocate space on the stack for local variables, if >> any; e.g. subl $n, %esp; >> (4). then follows the function body; >> to return from the function i would: >> (1). restore the stack pointer; e.g. movl %ebp, %esp; >> (2). restore the base pointer, e.g. popl %ebp; >> (3). then return to the calling function; >> >> i searched the 9fans archives for posts on assembly programming under >> plan 9; found some bits and pieces; e.g. in one of the posts it was >> mentioned that BP is a general purpose register, not the base pointer; and >> that FP is what ebp is under unix/linux; >> >> in the paper for the plan 9 assembler, it says that there are three >> registers available to manipulate the stack, FP, SP, and TOS; would the >> following comparison stand true then? >> plan9 unix/linux >> ------- ------------- >> FP EBP >> SP -4(%EBP)...-n(%EBP) /* local variables */ >> TOS ESP >> >> thanks; >> >> sasha kapshuk >> >> > thanks; i'll look into that;
