Silicon Disc

[Tony Brewer]

To avoid a lot of irrelevant comments in the Speculator topic, I've created a new one for the Silicon Disc, a 256 kilobyte RAM drive I designed in 1986. I have enough original PCBs to assemble and sell more than 100 of these. However my Einstein and monitor are stored in the loft and due to more urgent issues I won't be able to look at this until November. In the meantime, I've moved the photo of the Silicon Disc here.

Tatung Einstein Silicon Disc board photo

ESD.jpg (205.03 KiB) Viewed 7016 times

[Tony Brewer]

Very impressed to see that you've implemented drive mapping so the Silicon Disc can be configured as Drive 0.

"Provision has been made in the silicon disc ROM for logical (DOS)
disc drive to physical (MOS) disc drive mapping. The routine that
accomplishes this is the patched DOS drive select routine ZSLDSC.
This routine uses a special silicon disc variable at address
FDFFH to determine which physical drive corresponds to which
logical drive. This variable is interpreted in the following way
<snip>

Thanks to everyone for the kind welcome here. Can I ask where you found the info you quoted? Web searches for Tatung Einstein Silicon Disc produce almost nothing, so it's a good job I know a bit about it!

Yes, I had to search hard for any information. Until this discussion, I was not even aware that anyone had developed a Silicon Disc for the Einstein.

The quote is taken from a document posted in the Silicon Disc folder in the Files section of the Tatung Einstein Computer Group. Access requires subscription and membership of the Tatung Einstein Groups.io account.

I found the original WordStar file I wrote with the quoted info and converted it to plain text keeping fully justified text. Here it is in a code block and as .zip file attachment:

Tatung Einstein Silicon Disc Programming

Code: Select all

                     EINSTEIN SILICON DISC

The  Einstein silicon disc consists of 256K of RAM,  arranged  as 
2048  sectors of 128 bytes each.  The RAM is port-mapped and thus 
is  not directly accessible as program memory.  The  input/output 
ports used by the silicon disc are given below.

          Port           Use

          XXF8H          Output low byte of sector number
          XXF9H          Output high byte of sector number
          NNFAH          Input or output data byte

[NN = 00H-7FH, XX = irrelevant.]

Before reading or writing data,  it is necessary to specify which 
sector is being accessed by outputting the sector number, e.g

          LD DE,0        Select sector 0
          LD C,F8H
          OUT (C),E      Output sector no. low
          INC C
          OUT (C),D      Output sector no. high

[The order of outputting the sector number is irrelevant.]

When  reading or writing data,  address lines A8-A14 are used  to 
specify one of the 128 bytes in the sector.  Given below are read 
sector  and write sector routines that are essentially  identical 
to those in the silicon disc ROM. Both routines assume the sector 
number has already been output.

READ:     LD HL,BUFF     HL -> sector buffer
          LD BC,7FFAH
          INIR           Read first 127 bytes
          INI            Read last byte
          RET

WRITE:    LD HL,BUFF     HL -> sector buffer
          LD BC,80FAH
          OTIR           Write 128 bytes
          RET

The following points should be noted.  The value in register B is 
used  both to define the byte within the sector and as a counter. 
The OUTI and OTIR instructions decrement the value in register  B 
before  placing  it on A8-A15,  whereas the INI and INIR do  not. 
Thus,  the first byte in the sector is accessed using port  7FFAH 
and  the  last byte using port 00FAH.  Although this is  how  the  
silicon  disc  software operates,  bytes may be read  or  written 
individually, and in any order, if desired.

The silicon disc ROM patches the following MOS MCAL routines,  so 
that the silicon disc may be accessed as drive 3.

     Number    Name           Description

     A2H       ZRSECT         512-byte sector read
     A3H       ZWSECT         512-byte sector write
     A4H       ZRBLK          Block 512-byte sector read
     A5H       ZWBLK          Block 512-byte sector write
     B6H       ZSLDSC         Select disc drive (DOS)
     BAH       ZRD128         128-byte sector read (DOS)
     BBH       ZWR128         128-byte sector write (DOS)

A description of the changes to each MCAL routine is given below.

ZRSECT

If  byte at FB50H (HSTDSC) = 3,  then read 512-byte  sector  from 
silicon disc, else do normal read.

ZWSECT

If  byte  at FB50H (HSTDSC) = 3,  then write 512-byte  sector  to 
silicon disc, else do normal write.

ZRBLK

If  value in register A = 3,  then read block of 512-byte sectors 
from silicon disc, else do normal block read.

ZWBLK

If value in register A = 3,  then write block of 512-byte sectors 
to silicon disc, else do normal block write.

ZSLDSC

This is the DOS select disc routine kept in ROM for  convenience. 
The changes to this routine are explained later.

ZRD128

This  is the DOS sector read routine kept in ROM for convenience. 
If  byte  at FB7DH (DISC) = 3,  then read  128-byte  sector  from 
silicon disc, else do normal read.

ZWR128

This is the DOS sector write routine kept in ROM for convenience. 
If  byte  at  FB7DH (DISC) = 3,  then write  128-byte  sector  to 
silicon disc, else do normal write.

Provision has been made in the silicon disc ROM for logical (DOS) 
disc drive to physical (MOS) disc drive mapping. The routine that 
accomplishes this is the patched DOS drive select routine ZSLDSC. 
This  routine  uses  a special silicon disc variable  at  address 
FDFFH  to  determine which physical drive  corresponds  to  which 
logical drive. This variable is interpreted in the following way

Bits 0,1 give physical drive number for logical drive 0
Bits 2,3 give physical drive number for logical drive 1
Bits 4,5 give physical drive number for logical drive 2
Bits 6,7 give physical drive number for logical drive 3

The default value is E4H or 11100100B,  which gives a logical-to-
physical mapping of 0-0, 1-1, 2-2 and 3-3.

The  main  reason for having disc drive mapping is  so  that  the 
silicon  disc  can be configured as drive 0 when using  the  DOS, 
making programs which refer to drive 0 when loading, for example, 
do so much more quickly.

[Note that the silicon disc is always drive 3 when using the MOS. 
To  make the silicon disc any other MOS drive requires a  silicon 
disc ROM alteration.]

The following code makes the silicon disc act as logical drive 0, 
physical drive 0 act as logical drive 1,  physical drive 1 act as 
logical  drive  2 and physical drive 2 act as  logical  drive  3. 
Alternatively, the MOS 'M' command could be used.

          LD A,93H
          LD (FDFFH),A

Logical-to-physical  drive  mapping is purely a software  feature 
operating within the DOS and requires no hardware changes to  any 
floppy disc drive.

When  the silicon disc is logical drive 0,  it may be found  that 
physical  drive 0 is accessed when a program is loaded or exited. 
This is because the DOS warm-boot routine loads the DOS from  MOS 
(physical)  drive  0,  then selects DOS (logical)  drive  0.  The 
solution  is  to patch the DOS warm-boot routine to load the  DOS 
from  the silicon disc,  after copying the DOS onto  the  silicon 
disc.

Code that copies the DOS from the system tracks of physical drive 
0  to  the  silicon  disc is given below.  The  MOS 'R'  and  'W' 
commands may be used instead.

          LD BC,0        Track 0, sector 0
          LD DE,5A00H    End address for DOS
          LD HL,4000H    Start address for DOS
          PUSH BC
          PUSH DE
          PUSH HL
          XOR A          Physical drive 0
          RST 8
          DB A4H         Read block
          POP HL
          POP DE
          POP BC
          LD A,3         Physical drive 3
          RST 8
          DB A5H         Write block

The code in the warm-boot routine for DOS version 1.31 that loads 
the DOS from disc is given below.

0000H:    JP FA03H       Jump to BIOS warm-boot jump

FA03H:    JP FAC9H       Jump to warm-boot routine

FAC9H:    LD SP,0100H    Reset stack
          LD HL,E100H    Start address for DOS to load
          LD DE,EC00H    End address for DOS to load
          XOR A          Physical drive 0
          LD B,A         Start at sector 0
          LD C,A         Start at track 0
          RST 8
FAD6H:    DB A4H         Load DOS (MCAL routine ZRBLK)

The silicon disc ROM contains a new MCAL routine, function number 
FFH,  that  simply calls ZRBLK after loading register A with  the 
value 3. Therefore, to load the DOS from silicon disc, it is only 
necessary  to  change the MCAL number A4H to FFH.

The following code patches DOS version 1.31.  The MOS 'M' command 
could be used if wished.

          LD A,FFH
          LD (FAD6H),A

Note  that the address of the warm-boot routine and the start and 
end  addresses for the DOS to load will vary from one version  of 
the  DOS  to another. Below is a program that patches  the  warm-
boot routine which works for all DOS versions. It also copies the 
DOS  from  physical drive 0 to the silicon disc,  which  it  then 
makes  logical drive 0.  The program is relocatable and should be 
executed  from drive 0.  If the program is run from  the  silicon 
disc,  an  error  will occur when the program  warm-boots,  since 
logical  drive  3 has been mapped to  physical  drive  2,  unless 
physical drive 2 is present.

Firstly, copy DOS.

          LD BC,0
          LD DE,5A00H
          LD HL,4000H
          PUSH BC
          PUSH DE
          PUSH HL
          XOR A
          RST 8
          DB A4H         Read DOS from physical drive 0
          POP HL
          POP DE
          POP BC
          LD A,3
          RST 8
          DB A5H         Write DOS to silicon disc

Now patch warm-boot routine.

          LD HL,(0001H)  HL -> BIOS warm-boot jump
          INC HL         Skip 'JP'
          LD E,(HL)
          INC HL
          LD D,(HL)      DE -> warm-boot routine
          EX DE,HL       HL -> warm-boot routine
          LD BC,0
          LD A,CFH
          CPIR           Search for 'RST 8'
          LD (HL),0FFH   Change A4H to FFH

Finally, make silicon disc logical drive 0.

          LD A,93H
          LD (FDFFH),A
          RST 0

A hex dump for this program is given below.

          01 00 00 11 00 5A 21 00
          40 C5 D5 E5 AF CF A4 E1
          D1 C1 3E 03 CF A5 2A 01
          00 23 5E 23 56 EB 01 00
          00 3E CF ED B1 36 FF 3E
          93 32 FF FD C7

The  above program can easily be made into a .COM file by loading 
the  hexadecimal values from address 0100H onwards using the  MOS 
'M' command and then saving on disc using the DOS 'SAVE' command. 

As  mentioned earlier,  the silicon disc consists of 2048 sectors 
of 128 bytes each,  the DOS sector size.  The MOS sector size  is 
512  bytes,  so  there  are four DOS sectors for  every  one  MOS 
sector.  Both DOS and MOS have the same number of bytes per track 
(5K),  comprising 40 DOS sectors per track or 10 MOS sectors  per 
track.  The relationship between silicon disc number and DOS and 
MOS  track and sector numbers as used in the silicon disc ROM  is 
given below.

    Sector           DOS                 MOS
    number     Track     Sector    Track     Sector

       0         0         0         0         0 (1st 128 bytes)
       1         0         1         0         0 (2nd 128 bytes)
       2         0         2         0         0 (3rd 128 bytes)
       3         0         3         0         0 (4th 128 bytes)
       4         0         4         0         1 (1st 128 bytes)
       .         .         .         .         .
       .         .         .         .         .
      39         0        39         0         9 (4th 128 bytes)
      40         1         0         1         0 (1st 128 bytes)
       .         .         .         .         .
       .         .         .         .         .
    2047        51         7        51         1 (4th 128 bytes)

Below is a routine that outputs the sector number of a particular 
DOS  sector and track,  given by the values in registers D and  E 
respectively. Sector and track values are assumed to be valid.

          LD L,E
          LD H,0         HL = track
          ADD HL,HL      HL = track * 2
          ADD HL,HL      HL = track * 4
          ADD HL,HL      HL = track * 8
          LD B,H
          LD C,L         BC = track * 8
          ADD HL,HL      HL = track * 16
          ADD HL,HL      HL = track * 32
          ADD HL,BC      HL = track * 40
          LD C,D
          LD B,0         BC = sector
          ADD HL,BC      HL = track * 40 + sector
          LD A,L
          OUT (F8H),A    Output sector number low
          LD A,H
          OUT (F9H),A    Output sector number high
          RET

The  above  code can easily be modified for MOS sectors  by  left 
shifting the MOS sector number twice before entering the routine.

Any  of  the MCAL disc read or write routines  mentioned  earlier 
(ZRSECT,ZWSECT,ZRBLK,ZWBLK,ZRD128,ZWR128) may be used to transfer 
data to or from the silicon disc. It is not necessary to call the 
select  disc drive routine (ZSLDSC) beforehand since the  silicon 
disc is not physically 'selected'. 


                           *    *    *


When  the  Einstein  is first switched on with the  silicon  disc 
connected,  the silicon disc is formatted by filling every sector 
with E5H. When the Einstein is thereafter reset, the silicon disc 
ROM  reads the last sector (512 bytes) on track 1  and,  provided 
every  byte is E5H,  it assumes that the silicon disc is  already 
formatted. Therefore,  this track is reserved and data should not 
be written to it.


                           *    *    *


The  current  version of the silicon disc software (version  1.1) 
uses only 1.5K of the available 8K in the EPROM.  The size of the 
DOS is exactly 6.5K,  so it is possible in theory to include  the 
DOS in the silicon disc EPROM.  Alternatively,  the silicon  disc 
routines could be merged with other routines into one EPROM (upto 
a maximum of 16K) to fit into the one spare ROM socket inside the 
Einstein.

[It  may be possible to increase the number of spare ROM  sockets 
available in the Einstein.]


                           *    *    *


The  silicon disc patches only those MCAL routines needed by  the 
silicon disc but could patch ALL of the MCAL routines and  create 
new  ones.  The  spare bytes in the EPROM might be used  to  hold 
these new or modified routines.


                           *    *    *


Copyright (c) J.A.Brewer 1986.

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[deanb01]

Hi Tony,

1986 eh? This looks very similar to the ones we used at Software Creations in Manchester. We used Tatungs as dev machines for edit and assembly. The ram drive was a godsend. I'm currently using a Tatung Einstein in a similar capacity archiving old source disks from back then, and recreating the development process we used. If and when you get around to selling these, I'd be very interested in purchasing one from you.