;Convert Allocation Vector and Version Number Rsx Program
;Morrow Designs, Inc.
;Namdar Bolour
;3-Aug-84

;--------------------------------------------------------------------------
;Note:  This RSX has been quickly pasted together by combining the code of
;the Allocation Vector Rsx with the code of the Version number Rsx, so
;the comment contents and placements may be a little off.
;
		CSEG
		.z80
		.Radix 16
Include		SYSEQU		;System constants definition file.

;-----------------------------------------------------------------------------
;Equates.
;
AlvMax		Equ	460	;# of bytes needed for new 1-bit allocation
				;vector - room for exactly 35 Mb disk (after
				;formatting) with 4k blocks:  (35 Mb/drive) *
				;(1024 bytes/kb) / (4 kb/block) /
				;(8 blocks/byte) = 1120 bytes per drive.

DsmOffset	Equ	5	;Offset from beginning of Dpb to Dsm word.

;---------------------------------------------------------------------------
;Standard Rsx Prefix area:  contains parameters and data areas used by CP/M.
;
	DB	22, 16, 0, 0, 0, 0    ;6-byte dummy serial # required by CP/M.
		Jp	Start	;Jump to Rsx body.
NextRsx:	DB	0c3	;'Jp' op-code required by CP/M.
		DS	2	;Space for address of next Rsx.
		DS	2	;Space for address of previous Rsx.
		DB	0	;Remove Rsx flag: No, DO NOT REMOVE during
				;next warm boot.
		DB	0	;Load this Rsx only if operating under non-
				;banked CP/M 3.0 flag: No, load it regardless
				;of banked or non-banked.
		DB	'CPM2    '	 ;Any 8-character name.
		DS	3	;Working area required by CP/M.

;-----------------------------------------------------------------------
;See if Bdos function 27 or 12 or neither:
;
Start:		Ld	(SaveA), A	;Preserve A in case not 27 or 12.
		Ld	A, C
		Cp	GetAlv
		JR	Z, AllocVec	;This is function 27.

		Cp	GetVer
		JR	Z, Version	;This is function 12.

		Ld	A, (SaveA)	;Neither 12 nor 27:  Restore A
		JR	NextRsx		;and go to Bdos.

;---------------------------------------------------------------------------
;
;Function:
;---------
;
;Rsx to intercept Bdos Get Allocation Vector function 27 and convert the
;CP/M 3.0 Allocation Vector (Alv) into a CP/M 2.2 Allocation Vector.
;Since the 3.0 Alv differs from the 2.2 Alv, programs written under 2.2
;that access the Alv do not work properly under 3.0.  To correct this
;problem, attach this Rsx to the .COM file that won't work, and this Rsx
;will make 3.0's Alv look like 2.2's.
;
;Discussion:
;-----------
;
;3.0 and 2.2 Alv's differ in only 2 ways:
;
;    1)	3.0's Alv is in the system bank.
;
;    2)	2.2 uses 1 bit per disk block to show a block's status, but 3.0 uses
;	2 bits.
;
;	One bit, call it the Temporary Bit, is used exactly as in 
;	2.2:  it is 0 if the block is free, and 1 if it is allocated. During
;	disk writes, these bits are set to 1 by Bdos as new blocks are
;	reserved, whether or not the directory entry using the blocks
;	has yet been permanently recorded on disk by a Close operation or by
;	the opening of a new file extent by Bdos.  We may call this
;	collection of bits the Temporary Alv too.
;
;	The other bit, call it the Permanent Bit, has no counterpart in 2.2.
;	It's 0 if the block is free, but it's set to 1 by Bdos only if the
;	directory entry using the block has been written to disk by a
;	Close operation or by the opening of new file extent by Bdos.  Thus it
;	represents the state of the block as it is permanently recorded
;	on disk.  We may call this collection of bits the Permanent Alv.
;
;	One can imagine that the 2 bits are identical just after a Bdos
;	Close operation or an opening of a new file extent, and that they
;	become different as disk blocks are reserved during disk writes,
;	until the next Close operation or opening of a new file extent.
;
;	How are all these bits organized in the Alv of 3.0?  The first set
;	of contiguous bits consists of the Temporary Bits, in the same 
;	order as 2.2.  Following this set are the Permanent Bits, in the same
;	order as, and corresponding one-to-one with, the first set of bits.
;	This second set of bits starts with the byte following the 
;	last byte of the Temporary Bits.
;
;	I have discovered this organization by watching the 3.0 Alv change
;	as files are added but not closed.  The organization of 3.0's new
;	Alv is not documented in the CP/M manuals, and so this description
;	is not officially from Digital Research, and DR may change this
;	organization in the future.
;
;Program Operation:
;------------------
;
;Difference 2 is reconciled by looking at just the first set of bits in the
;3.0 Alv, the Temporary Bits, which are just like 2.2's, and ignoring the
;Permanent Bits.
;
;Difference 1 is reconciled by simply moving the Temporary Bits of the Alv
;from the system bank into the Tpa bank.
;
;Thus, this program operates as follows:
;
;    1)	Calculates how many Alv bytes to move from the system bank to Tpa 
;	bank by dividing the Dsm word in Dpb by 8 and adding 1.
;
;    2)	Gets Alv's start address (in system bank).
;
;    3) Uses Morrow Extended Bios function 0 to move the first part of
;	of the Alv, the Temporary Bits, from the system bank into
;	the Tpa bank.
;
;    4) Passes back in HL the start address of area in Tpa where
;	Alv was moved.

;--------------------------------------------------------------------------
;Start a new stack and save the old SP.
;
AllocVec:	Ld	HL, 0
		Add	HL, SP		;Get old stack in HL.
		Ld	SP, NewStack	;Make new stack.
		Push	HL		;Save old SP on new stack.

;-------------------------------------------------------------------------
;On the stack, put how long the Temporary Alv will be (in bytes).
;This count is determined by examining the Dsm word in Dpb.
;
DoRsx:		Ld	C, GetDpb	;Get this drive's Dpb address into
		Call	NextRsx		;HL by calling Bdos.

		Ld	DE, DsmOffset	;Get this drive's
		Add	HL, DE		;Dsm address into HL.

		Ld	E, (HL)		;Get Dsm
		Inc	HL		;word into
		Ld	D, (HL)		;DE.

		Ld	B, 3
DivideBy2:	SRL	D
		RR	E		;Divide Dsm by 8 to get # of bytes
		DJNZ	DivideBy2	;in Temporary Alv.

		Inc	DE		;Add 1 in case there was remainder.
		Push	DE		;Save # of bytes in Temporary Alv
					;on stack, for use later.

;-----------------------------------------------------------------------
;Get CP/M 3.0 Alv address (in system bank) into HL.
;
		Ld	C, GetAlv
		Call	NextRsx		;Call Bdos.

;------------------------------------------------------------------------
;Transfer first section of Alv from system bank into Tpa bank.
;
		Pop	BC	;Get # of bytes in Temporary Alv off
				;the stack (this is also the # of bytes to 
				;transfer from system bank).

	;;;Add only this instruction to get address of Permanent Alv.
	;;;There is nothing analogous to this in CP/M 2.2.
	;;;	Add	HL, BC

		Ld	DE, Alv
		Xor	A		;A=0:  Read System Memory function.
		Call	ExtendedBios	;Transfer Temporary Alv.

;------------------------------------------------------------------------
;All done.  Restore old SP.  Return in HL address of new 1-bit Allocation
;Vector.
;
		Pop	HL		;Restore
		Ld	SP, HL		;old SP.

		Ld	HL, Alv		;Return start address of new Alv.
		Ld	B, H		;All Bdos functions must return
		Ld	A, L		;with B=H and A=L!!

		Ret		;Don't go to Bdos - return to Bdos caller.

;==========================================================================

;---------------------------------------------------------------------------
;Rsx to intercept Bdos Return Version Number function 12 and return 2.2
;instead of 3.1.  Attach this Rsx to programs written under 2.2 which 
;will not run under 3.0 because they check for version # 2.2.  This Rsx
;will not insure that such programs will run correctly under 3.0 - it only
;enables them to run under 3.0.  Be sure that there are no other
;incompatabilities.

;---------------------------------------------------------------
;Return version no. 2.2:
;
Version:	Ld	HL, 22
		Ld	B, H		;All Bdos functions must return
		Ld	A, L		;with B=H and A=L!!
		Ret		;Do not go to Bdos - return to caller.

;--------------------------------------------------------------------
;Subroutine ExtendedBios:
;------------------------
;
;This is a tricky way to call the Morrow Extended Bios functions with 
;parameters in all the 8080 registers, including HL (!), quickly and in a 
;minimum number of bytes.
;
;On entry:
;	A selects the particular Extended Bios Function.
;	BC = # of bytes to move from system bank to Tpa bank, if applicable
;		to the particular function.
;	DE = Destination address of data to move, if applicable.
;	HL = Source address of data to move, if applicable.
;
;On exit:
;	The Extended Bios Function has been performed by the Bios, not
;	this subroutine.
;
;Registers preserved:	IX, IY.
;
ExtendedBios:	Push	HL		;Save param. to this subroutine.
		Ld	HL, (1)
		Ld	L, 0		;HL=Bios cold boot address.
		Ex	(SP), HL	;Put param. back in HL, cold boot
					;addr. on stack.
		Ret		;This pops stack to go to cold boot and
				;perform Extended Bios function, not to
				;return to the caller of this subroutine.
				;Ret at end of Extended Bios function will
				;return to caller.

;--------------------------------------------------------------------
;Data Storage Areas
;
SaveA:		DS	1

Alv:		DS	AlvMax, '&'	;Local space, in Tpa bank, for 2.2-
					;compatible Alv.  Filled with & for
					;testing purposes only.

		DS	10		;New stack area (8-level).
NewStack:

		END