User:Johnburger/Demo/Exec/Alloc/RAM: Difference between revisions

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This does what it says: zeroes the passed-in number of bytes starting from the passed-in pointer.
This does what it says: zeroes the passed-in number of bytes starting from the passed-in pointer.


== <code>[[(削除) JohnBurger (削除ここまで):Demo/Demo|Demo]]/[[(削除) JohnBurger (削除ここまで):Demo/Exec/Exec|Exec]]/[[(削除) JohnBurger (削除ここまで):Demo/Exec/Alloc/Alloc|Alloc]]/RAM.inc</code> ==
== <code>[[(追記) User (追記ここまで):(追記) Johnburger/ (追記ここまで)Demo/Demo|Demo]]/[[(追記) User (追記ここまで):(追記) Johnburger/ (追記ここまで)Demo/Exec/Exec|Exec]]/[[(追記) User (追記ここまで):(追記) Johnburger/ (追記ここまで)Demo/Exec/Alloc/Alloc|Alloc]]/RAM.inc</code> ==
<source lang="asm">
<source lang="asm">
;
;

Latest revision as of 09:27, 29 August 2017

This module contains two functions:

Exec.Alloc.RAM

This function allocates the passed-in number of bytes, and returns its linear address. Note that this RAM is not yet accessible: you need a Descriptor for that! But at least you now have the Base and Size parameters to create one.

Please note that this is a naïve implementation: it merely adds the requested size to the current top of RAM, and returns the previous value. It is intelligent enough to jump the 640 kiB to 1 MiB device/ROM/BIOS hole, but it doesn't even know where the ultimate top of RAM is! That has been left as an exercise for the reader...

Exec.Alloc.RAM.Zero

This does what it says: zeroes the passed-in number of bytes starting from the passed-in pointer.

Demo/Exec/Alloc/RAM.inc

;
; Exec/Alloc/RAM.inc
;
; This module provides RAM-related functions, such as Allocating and Zeroing
; blocks of RAM. It could use the RAM Map discovered from the BIOS during Boot,
; but that's been left as an exercise for the reader!
;
; Instead, the allocator merely keeps track of the last allocated address as a
; "high water mark", returning zero if that mark becomes greater than what's
; available. Of course, this naïve implementation merely keeps giving more and
; more - with enough intelligence to skip from RAMTop to the 1 MiB mark.
;...............................................................................
; This function allocates a block of RAM from the available pool.
; Note that it doesn't allocate a Descriptor for it, since it doesn't know
; what type it'll be, nor which Table to put it in.
;
; Also note that this implementation is NOT thread-safe. It assumes that only
; one Task will ever be accessing it at the same time. To work around that,
; either some kind of lock will be required, or atomic operations such as used
; in the 80486 (such as XADD) should be used.
;
; Input: DS = Data
; ECX = Size requested
; Output: EAX = Address of allocated RAM, or zero if insufficient.
; ECX preserved
Exec.Alloc.RAM:
MOVEAX,[Data.RAM]; Get current high-water mark
.Retry:
LEAEDX,[EAX+ECX+0Fh]; Get Top, rounded to paragraph
ANDEDX,~0Fh; Well, it is now!
CMPEDX,[Data.RAMTop]; Too far?
JBE.Save; Not yet...
MOVEAX,0010_0000h; Skip to 1 MiB
MOVDWORD[Data.RAMTop],-1; And pretend the rest is ours!
JMP.Retry; And try again
.Save:
MOV[Data.RAM],EDX; Save new high-water mark
.End:
RET
;...............................................................................
; This function zeroes out the block of passed-in RAM.
; Input: ES = Segment to zero
; ECX = Size to zero
; EDI = Where to zero
; Output: ES and ECX preserved.
; EAX and EDI? Not so much!
Exec.Alloc.RAM.Zero:
PUSHECX; As promised...
CLD; Work forwards
XOREAX,EAX; Store zeroes
SHRECX,2; Do DWORDs initially
REPSTOSD; Zero out whole DWORDs at a time
MOVECX,[ESP]; Recover saved value
ANDECX,03h; Mask off DWORDs already done
REPSTOSB; Make up any last bytes
POPECX
RET
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