fix, in func36 shift down amount total clusters (picked)

Imported from
 - https://hg.pushbx.org/ecm/edrdos/rev/ec560d130824
 - https://hg.pushbx.org/ecm/edrdos/rev/df6931dc0c9a

Tracked at #129

Reference FreeDOS implementation:
https://github.com/FDOS/kernel/blob/5de2eb1aa01b0129b046cf06338453a233c6597b/kernel/dosfns.c#L838

This loop now shifts right both the 32-bit amount total clusters
and the 32-bit amount free clusters. Further, it takes into account
the sector size to determine if the returned cluster size (bps times
spc) is still below 32 KiB. Previously it only checked for an spc
below 64.

For large clusters (already > 32 KiB before the loop) the condition
of "fit within 32 bits" is not ensured by this revision yet.

drdos/disk.asm

@@ -439,7 +439,7 @@ fdos_DI_error:
 ;	*****************************
 ;
 	Public	func36
-func36:
+func36	proc
 	call	set_retry_RF		; Valid to RETRY or FAIL
 	xor	dh,dh			; clear out DH
 	call	fdos_DISKINFO		; find out about drive
@@ -455,56 +455,65 @@ f36_OK:
 	mov	cx,es:DDSC_SECSIZE[bx]	; Get the Physical Sector Size
 	call	return_CX		; in bytes
 
-	mov	dx,es:DDSC_NCLSTRS[bx]	; Convert the last cluster no
-	cmp	es:DDSC_DIRENT[bx],0	; FAT32 file system?
+	xor	si,si
+	mov	di,es:DDSC_NCLSTRS[bx]	; si:di = last valid cluster number
+	cmp	es:DDSC_NFATRECS[bx],0	; FAT32 file system?
 	 jne	f36_OK20		; no, then use the 16-bit value
-	mov	dx,es:word ptr DDSC_BCLSTRS[bx]	; Convert the last cluster no
-	cmp	es:word ptr DDSC_BCLSTRS+2[bx],0	; more than fits into 16-bit register?
-	 je	f36_OK20			; no, the value is exact
-	mov	dx,0ffffh		; yes, so fake the value for maximum compatibility
+	mov	si,es:word ptr DDSC_BCLSTRS+2[bx]
+	mov	di,es:word ptr DDSC_BCLSTRS[bx]	; si:di = last valid cluster number
 f36_OK20:
-	dec	dx			; returned in DDSC to maximum
-	call	return_DX		; number of clusters
-
-	mov	cx,es:DDSC_FREE[bx]	; get number of free clusters
+	sub	di,1
+	sbb	si,0			; si:di = amount of data clusters
 	xor	dx,dx
-	cmp	es:DDSC_DIRENT[bx],0	; FAT32 file system?
+	mov	ax,es:DDSC_FREE[bx]	; dx:ax = number of free clusters
+	cmp	es:DDSC_NFATRECS[bx],0	; FAT32 file system?
 	 jne	f36_OK25		; nah, it must be FAT12/16
-	cmp	es:DDSC_MEDIA[bx],8fh	; or is it a CD-ROM drive?
-	 je	f36_OK25		; apparently it is one, so handle it like FAT12/16
-	mov	cx,es:word ptr DDSC_BFREE[bx]	; get number of free clusters (32-bit)
 	mov	dx,es:word ptr DDSC_BFREE+2[bx]
+	mov	ax,es:word ptr DDSC_BFREE[bx]	; dx:ax = number of free clusters (32-bit)
 f36_OK25:
-	xor	ax,ax
-	mov	al,es:DDSC_CLMSK[bx]	; get the sectors per cluster -1
-	inc	ax			; AX = sectors per cluster
+	mov	cl,es:byte ptr DDSC_CLSHF[bx]	; cl = spc shift
 ifdef DELWATCH
-	add	cx,FD_ADJUST		; now add in DELWATCH adjustment
+	add	ax,FD_ADJUST		; now add in DELWATCH adjustment
 	adc	dx,0
 endif
 f36_OK30:
-	cmp	dx,0			; more than fits into 16-bit register?
-	 je	f36_OK50			; no, the value is exact
-	cmp	al,64			; cluster size already 64K?
-	 jae	f36_OK40
-	shl	al,1			; cluster size * 2
-	shr	dx,1			; free clusters / 2
-	rcr	cx,1
-	jmp	f36_OK30			; try again
+	test	si,si			; si:di < 10000h ?
+	 jz	f36_OK40		; yes --> (implies dx:ax < 10000h as well)
+	push	ax
+	mov	ax,es:DDSC_SECSIZE[bx]
+	shl	ax,cl			; adjusted cluster size >= 32 KiB ?
+	cmp	ax,32*1024		; NC (jae) if we want to break out of loop
+	cmc				; CY if we want to break out of loop
+	 jc	@@skip			; if to break out -->
+	cmp	ax,es:DDSC_SECSIZE[bx]	; CY if shift had overflow	
+@@skip:
+	pop	ax
+	 jb	f36_OK40		; break out of loop -->
+	inc	cx			; increase spc shift
+	shr	dx,1			
+	rcr	ax,1			; free clusters / 2
+	shr	si,1
+	rcr	di,1			; total clusters / 2
+	jmp	f36_OK30		; try again
 f36_OK40:
 	test	dx,dx			; more than fits into 16-bit register?
-	 je	f36_OK50			; no, the value is exact
-	mov	cx,0fffeh		; yes, use a sane value for compatibility's sake
-f36_OK50:
-;	xor	ax,ax
-;	mov	al,es:DDSC_CLMSK[bx]	; get the sectors per cluster -1
-;	inc	ax			; AX = sectors per cluster
-
-	mov	bx,cx
-	call	return_BX		; return free clusters
-
+	 jz	@@ax_valid		; no, the value is exact
+	 mov	ax,-1			; yes, use a sane value for compatibility's sake
+@@ax_valid:
+	test	si,si
+	jz	@@di_valid
+	mov	di,-1
+@@di_valid:
+	mov	dx,di			; dx = adjusted amount total clusters
+	call	return_DX		; return amount total clusters
+	xchg	bx,ax			; bx = adjusted amount free clusters
+	call	return_BX		; return amount free clusters
+	mov	ax,1
+	shl	ax,cl			; ax = adjusted spc
 f36_exit:
 	jmp	return_AX_CLC
+func36	endp
+
 
 	public	fdos_DISKINFO
 fdos_DISKINFO: