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

page ,132
title nmemmax - Find the size of biggest block on near heap
;***
;nmemmax.asm - Find the size of biggest block on near heap
;
; Copyright (c) 1988-1992, Microsoft Corporation. All rights reserved.
;
;Purpose:
; Find the size of biggest block on near heap
;
;*******************************************************************************


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

sBegin data
assumes ds,data

externW _nheap_desc ; near heap descriptor

sEnd data

externNP _scanseg ; scan a heap segment
externNP _growscan ; see how big a heap segment may grow
externNP _findlast ; find the last entry in a heap


sBegin code

assumes cs,code
assumes ds,data

page
;***
;_nmemmax - Find the size of biggest block on near heap
;
;Purpose:
; _nmemmax() scans the near heap and determines the size of
; the largest _nmalloc() block that can be allocated.
;
; [NOTE: This routine assumes that the near heap is initialized.]
;
;Entry:
;
;
;Exit:
; ax = size of biggest block
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************


;--- Single thread version

labelP

cProc _nmemmax,,<>


cBegin

push si ; save caller's si
push di ; save caller's di

; Scan the segment

ifndef _LOAD_DGROUP
mov bx,dataoffset _nheap_desc ; ds:bx = heap descriptor
endif ;_LOAD_DGROUP
mov ax,-1 ; don't calculate _nfreect
call _scanseg ; cx = size of biggest free block

; Take into account how big we can grow the segment.
; If last block is free, see if that block combined with the growth
; is bigger than the current winner.
; cx = size of biggest free block

;*** NOTE: We currently depend on _growscan NOT changing cx
;push cx ; save biggest block so far
call _growscan ; ax = # of unused bytes after heap
;pop cx ; restore biggest so far
or ax,ax ; potential growth = 0 ??
jz compare ; yes, skip over findlast code

push ax ; save ax
call _findlast ; si = pointer to last heap entry
pop ax ; restore ax
mov dx,[si] ; length of last entry
test dl,1 ; is it free ??
jz compare ; no, use ax as is
dec dx ; dx = length
add ax,dx ; ax = last block + potential growth
add ax,2 ; Only 1 header needed (growscan adds 2
; to account for potential header)
compare:
cmp ax,cx ; new size > old size ??
ja done ; yes, return new size
mov ax,cx ; no, return old size
;fall thru

;
; Return
; ax = return value
;

done:
pop di ; restore caller's di
pop si ; restore caller's si
ret

cEnd

sEnd code

end