DFS 2.45 patches for MOS 3.50

From the depths of my memory, the patches are to fix three problem areas:

• OSWORD &7D and &7E didn't work.
  They both resulted in the OSWORD &7F routine being called.
• When trying to use DFS after ADFS, a 'Disc Write Protected' error occurs.
  There is some mention of this bug in Beebug Magazine, Vol 10, Number 2, page 59.
• The (undocumented ?) feature that setting bit 3 of the drive number should make the DFS work in double density.

The patches I made many years ago are shown here. The Megabit ROM Offset is the offset into the 1 Megabit ROM (useful if you've used my BBC BASIC program to save the whole 1 Megabit ROM image to file). The BBC Address is the address in the BBC memory map (with ROM 9 active).

Megabit ROM Offset	BBC Address	Old byte	New Byte		Old Code	New Code
04CA1	8CA1	29	20		AND #&08	JSR &BF82
04CA2	8CA2	08	82
04CA3	8CA3	F0	BF		BEQ &8CA7
04CA4	8CA4	02	EA			NOP
04CA5	8CA5	A0	EA		LDY #&10	NOP
04CA6	8CA6	10	EA			NOP
Megabit ROM Offset	BBC Address	Old byte	New Byte		Old Code	New Code
051F4	91F4	A9	86		LDA #&FF	STX &C7
051F5	91F5	FF	C7
051F6	91F6	8D	84		STA &C282	STY &C8
051F7	91F7	82	C8
051F8	91F8	C2	A9			LDA #&FF
051F9	91F9	86	FF		STX &C7
051FA	91FA	C7	8D			STA &C282
051FB	91FB	84	82		STY &C8
051FC	91FC	C8	C2
Megabit ROM Offset	BBC Address	Old byte	New Byte		Old Code	New Code
0622C	A22C	A6	A4		LDX &F0	LDY &EF
0622D	A22D	F0	EF
0622E	A22E	86	EA		STX &C7	NOP
0622F	A22F	C7	EA			NOP
06230	A230	A6	A6 (No change)		LDX &F1	LDX &F0
06231	A231	F1	F0
06232	A232	86	86 (No change)		STX &C8	STX &C7
06233	A233	C8	C7
06234	A234	C8	A6		INY	LDX &F1
06235	A235	10	F1		BPL &A23E
06236	A236	07	86			STX &C8
06237	A237	A6	C8		LDX &C7
06238	A238	C7	C8			INY
06239	A239	A4	10		LDY &C8	BPL &A23E
0623A	A23A	C8	03
0623B	A23B	4C	4C (No change)		JMP &91F4	JMP &91F8
0623C	A23C	F4	F8
0623D	A23D	91	91 (No change)
Megabit ROM Offset	BBC Address	Old byte	New Byte		Old Code	New Code
07B69	BB69	8C	8C (No change)		STY &C2DE	STY &C2E6
07B6A	BB6A	DE	E6
07B6B	BB6B	C2	C2 (No change)
07B6C	BB6C	60	60 (No change)		RTS	RTS
07B6D	BB6D	AC	AC (No change)		LDY &C2DE	LDY &C2E6
07B6E	BB6E	DE	E6
07B6F	BB6F	C2	C2 (No change)
Megabit ROM Offset	BBC Address	Old byte	New Byte		Old Code	New Code
07F82	BF82	FF	29		(blank)	AND #&08
07F83	BF83	FF	08		(blank)
07F84	BF84	FF	F0		(blank)	BEQ &BF8A
07F85	BF85	FF	04		(blank)
07F86	BF86	FF	A9		(blank)	LDA #&20
07F87	BF87	FF	20		(blank)
07F88	BF88	FF	A0		(blank)	LDY #&10
07F89	BF89	FF	10		(blank)
07F8A	BF8A	FF	60		(blank)	RTS

The code change around &A22C is to sort out the OSWORD calls. The OSWORD call number is held in location &EF: the original code had copied it to the Y register before &A22C - and then corrupted the Y register. The code at &91F4 is reorganised to as to work with the code at &A22C.

The code around &BB69 is to do with Paged ROM Service Calls &0B (NMI released) and &0C (NMI claim) - the change is to the location where the previous NMI owner's ROM number was stored. Acorn had originally chosen a location which was also being used for holding the 'double stepping' flag for drive 0 (i.e. after the result of a *DRIVE 0 40). When double stepping is set for a drive, the DFS regards that drive as being write protected.

The code changes at &8CA1 and &BF82 fix a bug which prevented support for double density (MFM) mode. OSWORD &7F should work in double density mode (16 sectors/track rather than the 10 sectors/track in single density mode) if bit 3 of the drive number byte is set. I.e. drive numbers 0 - 3 work in single density, drive numbers 8 - 11 work in double density.
I'm not sure if this feature was ever documented. It was never present in the original Acorn DFS - simply because the 8271 FDC didn't support double density. It must have arrived in the Acorn 1770 DFS - either in the BBC B upgrade, or in the B++. When the bit allocation of the Drive Control Register was changed, the authors of the Acorn 1770 DFS perhaps didn't realise that the 'density select' bit had changed from bit 3 to bit 5 and that they needed to cope with this.
I'm presuming that some 3rd party disk software relied on this double density feature, which is why I noticed the problem. Perhaps this was the Advanced Disk Investigator ROM ?