BBC Master 128 and Y2000

The hardware of Acorn Computers' BBC Master 128 microcomputer includes a real-time clock. Unfortunately the Machine Operating System (MOS) only handles the real-time clock up to 1999. To quote page 26 of Acorn's "Welcome Guide" for the Master series: The CMOS RAM holds and maintains a perpetual calendar which caters for all dates (including leap years) until the year 2000. When the guide was written in 1986, the year 2000 probably seemed like a long way in the future. However, I'm writing this in 1999 and the year 2000 is pretty darn close.

The real-time clock integrated circuit in the BBC Master 128 is a 146818. From reading a Hitachi datasheet for their HD146818, the IC provides a one-hundred year calendar by storing just a 2-digit year. Furthermore, when the year number is a multiple of 4 a leap year is assumed (so we might have more trouble in the year 2100, which isn't a leap year).

Since the real-time clock IC doesn't know about the century, clearly it is Acorn's MOS which is assuming that the date is in the 20th century. I resolved to see what could be done to make the real-time clock work into the 21st century.

My objective was to come up with a patch to the MOS ROM. This could be implemented either by:

( There is a alternative approach to the problem. Doomsday is a commercial sideways ROM which tackles several issues, including the Y2000 problem ).

It doesn't seem possible to obtain an EPROM device which is a drop-in replacement for the 128K MOS ROM. The MOS ROM is a 128K × 8 (1 megabit) device in a 28-pin 'dual in line' (DIL) package, whereas the majority of 128K × 8 EPROMs (e.g. 27C010 ) are in 32-pin DIL packages.
It is possible to create an adaptor board to enable a 32-pin DIL 128K × 8 EPROM to be used with the 28-pin DIL MOS ROM socket on the main board. Then one needs to burn the 8 16K ROM images in the one EPROM. The ordering of the images is:
Megabit ROM OffsetRom NumberROM Name
00000-Operating System
0400091770 DFS
0800010Viewsheet
0C00011Edit
1000012BASIC
1400013ADFS
1800014View
1C00015Terminal

I believe that Vine Micros no longer make or support their Master OS Overlay Board. It was a popular add-on at the time, so you may already have one of them. If not, please contact me, because we might be able to have clones of the board made.

Armed with my trusty machine code monitor, I set to work to look at the OSWORD &0E (Read Real-time Clock) and &0F (Write Real-time clock) code.
Incidently, as part of this exercise I discovered the same documentation error in Acorn's Master Series Reference Manual - Part 1 (page D.3-25) and Adder Publishing's The New Advanced User Guide (page 356). When using OSWORD &0F to write the date and time, the format of the parameter block on entry should be: 

        XY             = 24
        XY+1 to XY+15  = <date string>
        XY+16          = 46 (i.e. ASCII code for ".")
        XY+17 to XY+25 = <time string>
Looking at the MOS code for handling the real-time clock, two observations were made:
  1. The code which writes the date to the real-time clock totally ignores the century supplied by the user.
  2. The code which converts a BCD clock value to a date/time string (i.e. the code used by OSWORD &0E with XY=0 and XY=2) always returns "19" as the century.
As a result of trying a few tests, I also discovered made an interesting observation regarding the real-time clock IC: This allows the real-time clock IC to be set to a date in any century. One can equally well set the real-time clock's date registers to Monday, 1 Jan 00 (e.g. 1900) or Saturday 1 Jan 00 (e.g. 2000).

With this information, there are several different possible ways to solve the 21st century problem:

  1. Leave the OSWORD &0F code alone, as it ignores the user-supplied century. Change the OSWORD &0E code to always return "20" as the century. This is a solution which works from 1 Jan 2000, but returns the wrong year information for 1999. It would be a good fix to apply from 2000.
  2. Leave the OSWORD &0F code alone, as it ignores the user-supplied century. Change the OSWORD &0E code, so that it returns "19" as the century for two digit years greater than a certain number (e.g. 80), and returns "20" for the century for years less than that number. This would return the correct year information for a one hundred year period (e.g. 1980 - 2079).
  3. Instead of storing the real 2 digit year number in the real-time clock IC, store it with an offset (e.g. '00' in the IC is used to represent 1980, '20' to represent 2000, and '99' to represent 2079). Provided the offset is a multiple of 4, the leap year calculation in the IC is not affected. This would need a change to the OSWORD &0F code, as it no longer could ignore the user-supplied century, and would need to subtract the offset. The OSWORD &0E code would need to be modified to determine the correct century and add the offset.
Whilst option 3 is technically neat, it needs the most modifications to the existing MOS. It also alters the meaning of the contents of the year register within the real-time clock, and so might cause problems with 3rd party software which directly reads the real-time clock IC. Option 3 has thus not been investigated.

There are (at least) two versions of the MOS ROM for the BBC Master 128. The original MOS ROM was version 3.20: some time later the alternative MOS ROM was made available, this was version 3.50
(Both versions had buggy DFS code, which no doubt helped the sales of the Vine Micros' overlay board.

).

Patches for the original MOS ROM (v3.20)

For solution 1, there is a single byte to change. This is at address &9881 in rom 15 (i.e. Terminal), which corresponds to offset 1D881 in the 1 Megabit ROM image. Its current value is &19; change this to &20 to change the century number (note that the value is the century number in BCD, not in decimal).

For solution 2, there is spare room in rom 15 for the patch code at the end of page &B8: if using the overlay board approach, this means one needs to burn a 16K ROM to overlay rom 15.
The patches are shown below. 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 15 active).
Megabit ROM OffsetBBC AddressOld byteNew ByteOld CodeNew Code
1D8809880A920LDA #&19JSR &B8E0
1D881988119E0
1D882988220B8JSR &9893
1D883988393EANOP
1D884988498EANOP
Megabit ROM OffsetBBC AddressOld byteNew ByteOld CodeNew Code
1F8E0B8E0FFC9(blank)CMP #&38
1F8E1B8E1FF38(blank)
1F8E2B8E2FFA9(blank)LDA #&19
1F8E3B8E3FF19(blank)
1F8E4B8E4FFB0(blank)BCS &B8E8
1F8E5B8E5FF02(blank)
1F8E6B8E6FFA9(blank)LDA #&20
1F8E7B8E7FF20(blank)
1F8E8B8E8FF4C(blank)JMP &9893
1F8E9B8E9FF93(blank)
1F8EAB8EAFF98(blank)

Patches for the Alternative MOS ROM (v3.50)

For solution 1, there is a single byte to change. This is at address &9664 in rom 15 (i.e. Terminal), which corresponds to offset 1D664 in the 1 Megabit ROM image. Its current value is &19; change this to &20 to change the century number (note that the value is the century number in BCD, not in decimal).

For solution 2, there is not room in rom 15 for the patch code. However there is plenty of spare room in the OS area in page &FB: if using the overlay board approach, this means one needs to burn 2 16K ROMs, to overlay rom 15 and the OS.
The patches are shown below. 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 15 active).
Megabit ROM OffsetBBC AddressOld byteNew ByteOld CodeNew Code
1D6639663A920LDA #&19JSR &FBF0
1D664966419F0
1D665966520FBJSR &9676
1D666966676EANOP
1D667966796EANOP
Megabit ROM OffsetBBC AddressOld byteNew ByteOld CodeNew Code
03BF0FBF0FFC9(blank)CMP #&38
03BF1FBF1FF38(blank)
03BF2FBF2FFA9(blank)LDA #&19
03BF3FBF3FF19(blank)
03BF4FBF4FFB0(blank)BCS &FBF8
03BF5FBF5FF02(blank)
03BF6FBF6FFA9(blank)LDA #&20
03BF7FBF7FF20(blank)
03BF8FBF8FF4C(blank)JMP &9676
03BF9FBF9FF76(blank)
03BFAFBFAFF96(blank)

Description of patch code

The code just prior to the 
LDA #&19
JSR ...
writes the 2 digit year from the real-time clock IC as a string in reverse order, so when it has finished writing the 2 digit year the accumulator holds the ASCII code for the decade number in BCD (e.g. &38 for the 80s, &39 for the 90s, and &30 for the 00s).
The patch tests whether the ASCII code for the decade number in BCD is 8 or 9: if it is, "19" in BCD is used for the century, otherwise "20" in BCD is used for the century.

This patch allows the year to be in the range 1980 - 2079. If the comparison at address &B8E0 (MOS 3.20)/&FBF0 (MOS 3.50) was made with #&39, then the year would be allowed in the range 1990-2089, giving an extra 10 years of operation.

Patching the MOS ROM

Provided that one patches one's existing OS ROM, I forsee no copyright problems. Acorn have not replied to an e-mail asking what their position is, which I take to mean that they're not interested.
One needs to download the existing code to a file, patch the file, and then burn a new EPROM (or two).

Downloading the existing code

If using the Vine Micros' Master OS Overlay Board , then one needs to download a copy of ROM 15 (Terminal) (for solutions 1 and 2). For solution 2 with MOS 3.50, one also needs to download the OS area.

If burning a whole new 128K × 8 EPROM, one needs to download a copy of ROMs 9-15 and the OS area.
The easiest solution is probably to use my BBC BASIC program to save the whole 1 Megabit ROM image to file.

If you want to download the individual sideways ROMs to files, there are many utilities to do this.
One also needs to download the OS area. It is slightly harder to download the OS area, because it is overlaid in two areas:

Using the Vine Micros' Master OS Overlay Board

For solution 1, one needs to save a copy of ROM 15 (Terminal), patch the single byte concerned, burn a new EPROM with this patched image, and put it into the Vine Micros' Master OS Overlay Board in a free socket. Jumper link 7 to the socket used.

For solution 2, follow the above steps, but there are more bytes to patch in ROM 15.
If working with MOS 3.50, one also needs to patch the OS area: save a copy of the OS area, patch the bytes, burn a new EPROM with this patched image, and put it into the Vine Micros' Master OS Overlay Board in a free socket. Jumper link 0 to the socket used.

Burning a new MOS ROM

What one needs to do is:
Information supplied by
Andrew Benham (G8FSL), Salisbury, Wilts SP1, United Kingdom