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Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!usenet.ins.cwru.edu!ncoast!dale
From: dale@NCoast.ORG (Dale Smith)
Newsgroups: comp.os.msdos.programmer,alt.msdos.programmer,comp.lang.c
Subject: Re: alloca() for TurboC
Keywords: alloca, DOS, TurboC, assembly
Message-ID: <1990Aug23.032751.15666@NCoast.ORG>
Date: 23 Aug 90 03:27:51 GMT
References: <2745@onion.reading.ac.uk> <3860@bingvaxu.cc.binghamton.edu> <1990Aug22.032243.18214@usenet.ins.cwru.edu>
Organization: North Coast Public Access *NIX, Cleveland, OH
Lines: 134

In article <1990Aug22.032243.18214@usenet.ins.cwru.edu> trier@po.CWRU.Edu writes:
>Here's a version of alloca I use.  It's my clone of a version from some
>piece of PD software I saw a year or two ago.
>
>        #define alloca(A) ((_SP -= ((A)+1) & 0xFFFE), (void far *)(((unsigned long)_SS << 16) | _SP))
>
>The idea is to subtract the size A from the stack pointer, making sure
>it's word-aligned.  (This is the (_SP -= ((A)+1) & 0xFFFE) part.)  Then,
>since alloca returns a pointer to the allocated space, the macro must
>kludge together a far pointer from the stack segment and stack pointer.
>
>I should add the disclaimer that I'm not entirely sure that this works.
>It's my own version of someone else's, but I changed it a bit.  (I don't
>remember what the original looked like.)
>
>Hope this helps!
>
>-- 
>Stephen Trier                              Case Western Reserve University
>Home: sct@seldon.clv.oh.us                 Information Network Services
>Work: trier@cwlim.ins.cwru.edu
>   I may work for the University, but that doesn't mean I speak for them.

I believe I saw something like that in an early port of bison.  It's
really cute but will trash register variables.

Someone posted an alloca for the 386 which I hacked to fit Turbo C.
I also took a peek at a disassembly of the Microsoft C 5.1 alloca().

Here it is, have fun.

------ Snip it or somethin' ------------------------------
; alloca.asm
; Allocate from the stack for Turbo C 2.0
; I have not tried this this TC++ 1.0

; void *alloca(size_t size);

; Beware of using alloca in functions without arguments
; and -k- in tiny, small, and medium models.  The stack has a
; possibility of growing.

    ifdef __TINY__
   .model   tiny
elseifdef __SMALL__
   .model   small
elseifdef __MEDIUM__
   .model   medium
elseifdef __COMPACT__
   .model   compact
elseifdef __LARGE__
   .model   large
elseifdef __HUGE__
   .model   huge
else
   .model small
endif


if @DataSize EQ 0
	.data
	extrn	___brklvl:word
endif

	.code
	public	_alloca
_alloca	proc

;; Pop return address into (es:) cx and size into ax
	pop	cx		; return address
if @CodeSize
	pop	es		; return segment
endif
	pop	ax		; size

;; Make sure that ax is greater than zero, and is a multiple of two.
	cmp	ax,0
	jl	error
	inc	ax
	and	ax,not 1

;; Figure new stack pointer, check for negative and less than brk.
	mov	bx,sp
	sub	bx,ax
	jb	error		; below zero
if @DataSize EQ 0		; tiny, small, medium
	cmp	bx,___brklvl
	jb	error		; below the brk
endif
	xchg	sp,bx
	mov	ax,sp


;; Copy the (hypothetical) register variables.
ifdef __MEDIUM__
	push	ss:6[bx]	; return segment(medium)
endif
	push	ss:4[bx]	; return offset(med,sm,tiny) or ds(huge)
	push	ss:2[bx]	; di (all)
	push	ss:[bx]		; si (all)

if @DataSize
	mov	dx,ss
endif

exit:
;; Put something on the stack so that our caller can pop it off.
	push	ax

;; Return to the the caller.
if @CodeSize
	push	es
	push	cx
	ret
else
	jmp	cx
endif

error:
	xor	ax,ax
if @DataSize
	cwd
endif
	jmp	exit

	endp
	end
-------------- Ok, that's it -----------------------------------------

dale
-- 
 Dale P. Smith
 dale@ncoast.org
 uunet!usenet.ins.cwru.edu!ncoast!dale