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+;; alloca386.s 1.2
+;; GNU-compatible stack allocation function for Xenix/386.
+;; Written by Chip Salzenberg at ComDev.
+;; Last modified 90/01/11
+;;> Is your alloca clearly better than the one in i386-alloca.s? I haven't
+;;> looked at either.
+;;
+;;They're different because Xenix/386 has a different assembler. SCO
+;;Xenix has the Microsoft C compiler and the Microsoft macro assembler,
+;;called "masm". MASM's assembler syntax is quite different from AT&T's
+;;in all sorts of ways. Xenix people can't use the AT&T version.
+;;--
+;;Chip Salzenberg at ComDev/TCT <chip@tct.uucp>, <uunet!ateng!tct!chip>
+
+ TITLE $alloca386
+
+ .386
+DGROUP GROUP CONST, _BSS, _DATA
+_DATA SEGMENT DWORD USE32 PUBLIC 'DATA'
+_DATA ENDS
+_BSS SEGMENT DWORD USE32 PUBLIC 'BSS'
+_BSS ENDS
+CONST SEGMENT DWORD USE32 PUBLIC 'CONST'
+CONST ENDS
+_TEXT SEGMENT DWORD USE32 PUBLIC 'CODE'
+ ASSUME CS: _TEXT, DS: DGROUP, SS: DGROUP, ES: DGROUP
+
+ PUBLIC _alloca
+_alloca PROC NEAR
+
+; Get argument.
+ pop edx ; edx -> return address
+ pop eax ; eax = amount to allocate
+
+; Validate allocation amount.
+ add eax,3
+ and eax,not 3
+ cmp eax,0
+ jg aa_size_ok
+ mov eax,4
+aa_size_ok:
+
+; Allocate stack space.
+ mov ecx,esp ; ecx -> old stack pointer
+ sub esp,eax ; perform allocation
+ mov eax,esp ; eax -> new stack pointer
+
+; Copy the three saved register variables from old stack top to new stack top.
+; They may not be there. So we waste twelve bytes. Big fat hairy deal.
+ push DWORD PTR 8[ecx]
+ push DWORD PTR 4[ecx]
+ push DWORD PTR 0[ecx]
+
+; Push something so the caller can pop it off.
+ push eax
+
+; Return to caller.
+ jmp edx
+
+_alloca ENDP
+
+_TEXT ENDS
+ END