/*
	e screen editor

	(C) G. Nigel Gilbert, MICROLOGY, 1981

	August-December 1981

	FILE: e9

	FUNCTIONS: initvm,freememslot,swapout,writepage,swappin,readpage,
			fatalerror

	PURPOSE: manages virtual memory

*/

#include "e.h"

initvm()	/*initialise virtual memory system*/
{
	sint slot;
	unsigned *cpm, base;

	cpm=6;	/*address of base of BDOS*/
	slotsinmem=(*cpm-7-TOP-(wboot?0:CCP))/PAGESIZE;
	if (slotsinmem < 3) {
		puts("More memory needed to run 'e'"); 
		exit();
		}
	for (slot=0, base=TOP; slot < slotsinmem; slot++, base+=PAGESIZE) {
		usage[slot]=FREE;
		slotaddr[slot]=base;
		}
	clock=0;

	/*reserve slot 0 for tp*/
	tp=TOP; 
	tppages=1;
	usage[0]=NOTAVAIL;

	/*force balloc to find a new page to start*/
	pageloc[(newpage=0)]=FREE; 
	allocp=PAGESIZE+1;

	/*paging file not yet created*/
	pagefd=NOFILE;
	strcpy(pagingfile,"  e$$$.$$$");
	if (pagingdisk) {
		pagingfile[0]=pagingdisk; 
		pagingfile[1]=':';
		}
}

freememslot()	/*returns the number of a free memory slot, possibly by swapping
		  out the least recently used page currently in memory*/
{
	int use, u;
	sint i, slot;

	for (use=MAXINT, i=0; use && i < slotsinmem; i++)
		if ( (u=usage[i]) != NOTAVAIL && u < use) {
			use=u; 
			slot=i;
			}
	if (use) /*no free slots*/ swapout(slot);
	return slot;
}

swapout(slot)	/*swaps page currently in memory at 'slot' to disk,
		  updating pageloc to show new location*/
sint slot;
{
	int *pl;

	/*find page number of page at 'slot'*/
	for (pl=&pageloc[0]; *pl != slot; pl++);
	*pl=-writepage(slot); /*update pageloc with disk slot written to */
}

writepage(slot)	/*writes page currently in memory at 'slot' to disk;
		  returns disk slot where written*/
sint slot;
{
	sint loc;

	if (pagefd == NOFILE) { /*haven't opened paging file yet*/
		if (dskcheck(setjmp(dskerr)) || (pagefd=creat(pagingfile)) == FAIL)
			fatalerror("Can't create a buffer file");
		for (loc=0; loc < MAXSLOTS; loc++) dskslots[loc]=FREE;
		}
	for (loc=0; dskslots[loc] != FREE; loc++); /*find a free slot*/

	if (dskcheck(setjmp(dskerr)) || seek(pagefd,loc*PAGESECTS,ABSOLUTE) == FAIL)
		fatalerror("Bad seek in writing buffer");
	if (write(pagefd,slotaddr[slot],PAGESECTS) != PAGESECTS)
		fatalerror("Can't write to buffer - disk full");
	dskslots[loc]=INUSE;
	usage[slot]=FREE;
	return loc;
}

swappin(page)	/*get 'page', currently on disk, into memory and return slot
		  where placed*/
sint page;
{
	sint slot;

	readpage( (slot=freememslot()), -pageloc[page]);
	usage[slot]=INUSE;
	return pageloc[page]=slot;
}

readpage(memslot,dskslot)	/*read a page from disk into memory at 'memslot'*/
sint memslot, dskslot;
{
	if (seek(pagefd,dskslot*PAGESECTS,ABSOLUTE) == FAIL)
		fatalerror("Bad seek in reading buffer");
	if (read(pagefd,slotaddr[memslot],PAGESECTS) != PAGESECTS)
		fatalerror("Can't read buffer");
	dskslots[dskslot]=FREE;
}

fatalerror(message)	/*displays message, returns to a known good screen*/
char *message;
{
	error(message);
	moveline(goodline-cline);
	longjmp(mainmenu);
}
least recently used page currently in memory*/
{
	int use, u;
	sint i, slo