BANKSWAP: 
	    A Banked Memory Debugging Tool for CP/M Plus

			 Albert S. Woodhull
			 Hampshire College
			 Amherst, MA 01002

		 Copyright 1985, Albert S. Woodhull

	The BANKSWAP.LBR file contains, in addition to this READ.ME
file, the source, BANKSWAP.ASM, and RSXMAKER.SUB, which can be used
to control the process of assembly, linking, and attaching the
resident system extension to an executable file.

	A complete description of the program was published in Dr.
Dobb's Journal, Volume 10, Issue 12, December 1985, on pages 34-52.
The following text is abstracted from that article:

	     BANKSWAP   is  not   a   standalone 
	program; it is an  enhancement to SID or 
	DDT that provides additional commands to 
	copy blocks of memory from bank to bank.  
	The normal functions of the debugger can 
	be used on a copy of memory from another 
	bank  that  has been brought to the  TPA 
	bank.
	
	     BANKSWAP is relocatable, and is not 
	necessarily loaded to the same  location 
	each  time it is used.   For ease of use 
	BANKSWAP  installs a vector to  its  own 
	entry  point at the RST 5 location (28H) 
	during installation.   Typing "G28" from 
	the  SID  or DDT prompt  brings  up  the 
	BANKSWAP   menu.    The   initialization 
	process   also  displays  a  message  to 
	remind  the  user  of  the  presence  of 
	BANKSWAP and the command to access it.
	
	     The  BANKSWAP menu allows choice of 
	the  direction of the move,  the  memory 
	addresses    for    the    source    and 
	destination, and the length of the block 
	to be moved.  The menu also provides for 
	easy return to DDT or SID,  and for  the 
	eventual removal of BANKSWAP. 
	
	     The listing contains comments  that 
	explain the operation of BANKSWAP, but I 
	will  emphasize  a  few points  I  found 
	important in working with banked memory.  
	Although I wrote this program for use on 
	an Apple with the A.L.S. CP/M Card there 
	should be no problems in making BANKSWAP 
	work  on other implementations  of  CP/M 
	Plus.  The  most critical point is to be 
	sure that control is not lost while bank 
	1  is deselected.  This  means  ensuring 
	that BANKSWAP itself, the stack, and all 
	data  areas  used are located in  common 
	memory.   It is possible for a CP/M Plus 
	system  to be constructed so  interrupts 
	and  system calls can be  handled  while 
	alternate banks are selected.   However, 
	for  insurance  I  thought  it  best  to 
	disable  interrupts  and avoid calls  to 
	the standard BDOS entry point while bank 
	1  is deselected,  since the vectors  on 
	page 0 of bank 1 are then inaccessible.
	
	     There  is a potential problem  with 
	the version of BANKSWAP shown in listing 
	1,  but  I leave it for someone else  to 
	fix.   As noted above, BANKSWAP uses the 
	RSX  technique in order to be located at 
	the highest available meory address. The 
	actual  amount  of high memory  that  is 
	common  depends upon the hardware  used, 
	and  I  didn't figure out a  way  for  a 
	program to determine this.   If too many 
	RSXs are installed, the top of available 
	memory  can be below the common  region.  
	In  this  case  BANKSWAP  will  probably 
	cause a crash.   On my Rev.  A CP/M card 
	the  common  memory limit is  at  8000H, 
	ahich leaves room for a lot of RSXs,  so 
	I  have never given the problem  a  high 
	priority. 

	The BANKSWAP.LBR file is being uploaded to the CompuServe
CP/M SIG on December 15, 1985.  The author can be reached by EMAIL
to 74156,1067 on CompuServe, or by mail to the address at the
beginning of this file.  He would appreciate being informed of
further distribution of these files.