Category : C++ Source Code
Archive   : VCCRT1.ZIP
Filename : BMALLOC.ASM

page ,132
title bmalloc -- C runtime based heap allocation
;***
;bmalloc.asm - based heap allocation
;
; Copyright (c) 1985-1992, Microsoft Corporation. All rights reserved.
;
;Purpose:
; defines _bfree(), _bmalloc() - allocate/free memory from the
; based heap.
;
;*******************************************************************************


include version.inc
.xlist
include cmacros.inc
include heap.inc
.list

sBegin data
assumes ds,data

externW _bheap ; based heap linked list descriptor

ifndef _NOTCXX_
globalCP _pnhbBasedHeap, 0
endif

sEnd data

externNP _findseg ; find a heap segment
externNP _searchseg ; search a heap segment
externNP _growseg ; grow a heap segment


sBegin code
assumes ds,data
assumes cs,code

page
;***
;void _bfree(bseg, boff) - free block in the based heap
;
;Purpose:
; Free a block of memory in the based heap which was allocated
; by _bmalloc().
;
; [MTHREAD NOTE: Since the freeing of a heap entry is an
; atomic action, the heap lock is not needed. Note that if
; the RESET_ROVER code is implemented, _bfree() must lock/unlock
; the heap.]
;Entry:
; unsigned bseg = based heap segment
; unsigned boff = offset into heap segment
;
; bseg:boff = address of block to be freed
;
;Exit:
;
;Uses:
; ax,bx,cx,dx,es
;
;Exceptions:
;
;*******************************************************************************


cProc _bfree,,

parmW bseg ; based heap segment
parmW boff ; offset into based heap segment

cBegin

; Get address and make sure it's not NULL

mov es,[bseg] ; es:si = address
mov si,[boff]
mov cx,es ; is bseg == 0 ??
jcxz _bfend ; yup - just return
mov cx,si ; is boff == 0 ??
jcxz _bfend ; yup - just return
cmp cx,_NULLOFF ; is boff == _NULLOFF ??
je _bfend ; yup - just return


or byte ptr es:[si-2],1 ; free entry


_bfend: ; return

cEnd

page
;***
;_bmalloc(bseg, incr) - allocate a memory block in the based heap
;
;Purpose:
; Allocates a memory block of at least size bytes in the based heap.
; The block will be aligned for the storage of any type of object.
;
;Entry:
; unsigned bseg = based heap segment
; unsigned incr = size of memory block desired
;
;Exit:
; dx:ax = success = address of block (dx = segment, ax = offset)
; = failure = NULL
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************


;--- Single thread version
cProc _bmalloc,,


parmW bseg ; unsigned int based heap segment
parmW incr ; unsigned int allocation size

cBegin

ifndef _NOTCXX_
RetryBMalloc:
endif

;
; Make sure the requested size is not too big
;
push ds ; save dgroup
mov dx,[incr] ; cx = request block size
cmp dx,_HEAP_MAXREQ ; cx > maximum legal request size ??
ja error_rtn ; error

;
; Find the appropriate based segment
; dx = size
; ds = dgroup
;

mov bx,dataoffset _bheap ; based heap list header
mov ax,[bseg] ; segment to search for
call _findseg ; get the segment
jc error_rtn ; error, if we didn't find the segment

;
; Try to get a block from the existing segment
; dx = size
; ds:bx = based heap segment descriptor
;

mov cx,dx ; cx = block size
call _searchseg ; try for a block
jnc have_block ; success!!

;
; Try to grow the segment
; cx = size
; ds:bx = based heap segment descriptor
;

call _growseg ; try to grow the block
jc error_rtn ; error, could not grow

call _searchseg ; get the new block

;*** OPTIONAL VALIDITY CHECK
;jc _heap_toast ; *** oops, should always work ***
;***

;
; The block is in hand. Update the roverseg field in the based heap list
; header (analogous to what _fmalloc() does) and exit.
; ds:bx = based heap segment descriptor
; dx:ax = pointer to block (to be returned to the caller)
;

have_block:
mov si,ds ; stash selector in si
pop ds ; restore dgroup
mov di,dataoffset _bheap ; ds:di = based heap list header
mov word ptr [di].roverseg,bx ; update rover seg
mov word ptr [di].roverseg + 2,si
jmp short bdone ; return the result

;
; Error return. We will always get here if _bmalloc() is going to
; return _NULLOFF.
;

error_rtn:
pop ds ; restore dgroup

ifndef _NOTCXX_

if sizeC
mov cx, word ptr (_pnhbBasedHeap+2)
or cx, word ptr (_pnhbBasedHeap)
jz rtn_NULLOFF
else ;not sizeC
cmp (_pnhbBasedHeap), 0
je rtn_NULLOFF
endif ;not sizeC

push [bseg]
push [incr]
call (_pnhbBasedHeap)
add sp, 4

or ax, ax
jnz RetryBMalloc
rtn_NULLOFF:
endif ;not _NOTCXX_

mov ax,_NULLOFF ; Set dx:ax = bseg:_NULLOFF
mov dx,[bseg] ; Fall through to bdone.

;
; Common exit
; (dx:ax = return value)
;

bdone:

cEnd

sEnd code

end