Written by Herb Johnson and Dwight Elvey, last edit Feb 10 2008. My thanks to Dwight Elvey for his programs and his work to re-engineer, restore and program Dr. John Torode's & Digital Systems Inc.'s FDC-1 controller. More discussion of Dr. Torode's floppy controller is on this linked page.
Dwight Elvey wrote to me in Feb 2008, as below, about his work on the FDC-1 microcode and software. The work on this page is based on This image of a portion of the microcontroller from the schematic for the Digital Systems FDC-1 controller card from December 1976. My edits to Dwight's comments below are in []'s. - Herb Johnson
Dwight Elvey: I've manually decompiled [the microcontroller] ROMs such that [the instructions] references [signals] on the schematic. That was how I first got it working. Another fellow, Stan Sieler, had the schematics but nothing else. I used the schematics and my ROM decompile to understand how to use [the floppy controller].
[Then] I assembled some [8080] boot strap code on my PC, and toggled it into my IMSAI. This was just a simple serial [port code] loader. I wrote code to format the disk and then to write the boot code on the first sector for the boot loader; and then the routines for CP/M to use.
[Then] I got the basic CP/M running and then used my simple serial and the debugger to load the rest of the code. (PIP just doesn't do serial well on binary).
I have the decompiled ROM code here. One file is the format code and the other is the normal read/write code. Since Torode couldn't get it all within one ROM sequence, to format one had to select a jumper on the interface card. I ran the jumper to a switch I put on the back panel of the IMSAI. Format is more complex than just read or write. One has to have data for everything except the CRC values of an entire track. I figured this all from the decompiled [microcontroller ROM] code. - Dwight Elvey
The FDC-1 is a hard-coded microcontroller, with an interface to a seperate "host processor" card. The microcontroller has its own ROM programs, which are controlled through the interface by the host computer. More discussion of Dr. Torode's floppy controller is on this linked page.
The FDC-1 manual says the host processor interface is a DMA interface. The interface provides 8-bit addresses and strobes for IN or OUT or DEVICE ADDRESS; and there is an 8-bit data path. References on the host side, presumably for an 8080 based system, is to addresses 07FH for status and commands, 07DH amd 07EH for DMA address, among other uses of those addresses. 131 decimal bytes (83H) are transferred per sector. The bootstrap reads track 0, sector 1, into DMA addresses 00-7FH.
The FDC-1 microcontroller has an internal 8-bit instruction bus on which instructions appear, as read from the 3601 ROM chips. They are decoded by a series of decoder chips, or other logic, which reads this bus. Each set of microinstructions are "read" by one of these decoders. There is also a decoder for various conditions of microcontroller activity; that decoder is read by "jump" circuity and the microcontroller will "jump" if the condition occurs (or not) when the jump instruction for that condition is executed. - Herb Johnson
From reading Dwight's reconstructed microcode, and the schematics as Dwight did, I've listed below an "instruction set" for the FDC-1 microcode.I've named the specific instruction decoder chips and pins on the FDC-1, and spelled out the signals which Dwight abbreviated slightly. This information must be read with the FCD-1 microcontroller schematic to make sense. - Herb Johnson
;chips B4 and B3 (74161) execute the "jumps", instructions 0X and 4X.
;chip A6 (74150) decodes the conditions to be tested as X = 0 thru F as follows below.
;these signals are ANDed with the signals named and fed back to the microcontroller.
; the "/" means the signal is active low, otherwise it is active high.
; pin 8 D0 T (tied high)
; pin 7 D1 EQUAL
; pin 6 D2 DATA AM
; pin 5 D3 /IDAM
; pin 4 D4 CNT DONE
; pin 3 D5 /DATA OK
; 2 D6 BYT RDY
1 D7 /READY
23 D8 READ REQ
22 D9 WRITE REQ
; 21 D10 MAC RDA
; 20 D11 /TRK ZERO
; 19 D12 STP RDY
; pin 18 D13 /BOOTSTRAP (changed from RESET)
17 D14 /INDEX
pin 16 D15 (unused)
;4X instructions will jump on the condition (signal) given.
;0X instructions will jump on the NEGATION of these conditions.
;the subsequent microcode byte gives the 8-bit address to jump to.
;"J" is jump, "JN" jump on not
chips B3, B4, jump on negation of selected condition
01 JN(EQ)
02 JN(DAM)
03 J(IDAM)
04 JN(CNT)
06 JN(BRDY)
0A JN(MAC)
0C JN(SRDY)
0D J(RST) <<-- or BOOTSTRAP
chips B3, B4, jump on selected condition
40 JMP
43 JN(IDAM)
45 JN(OK)
46 J(BRDY)
47 JN(READY)
48 J(RD)
49 J(WR)
4A J(MAC)
4C J(SRDY)
These instructions send specific control signals from the microcontroller to the rest of the controller, or back through the interface to the host processor.
:Instruction 80 is a signal from chip C4 pin 8, a few TTL gates 80 LDCNT ;CX instructions are from chip B6, 3205 C0 SET1X C1 DCRDAT C2 SETSTP C3 CLRDMAD C4 CLRDAT C5 CNTDAT C6 DCRDMAD C7 INRDMAD ;FX instructions are from chip E1, 74154 F0 CLRCRC F1 WRCRC F2 SETCNTC F3 SETCNTD F4 CLRBRDY F5 SETCLK0 F6 CLRWG F7 SETWG F8 CLRDSR F9 SETTER FA SETDCE FB SETICE FC SETIOF FD DMARD FE DMAWR FF ZERODBIT
The read/write microcode instructions from the DS microcontroller are listed on on this Web page. These were named by Dwight Elvey using the signals named on sheet #1 of the DS FDC-1. I've slightly edited the file Dwight sent me as he noted above. - Herb Johnson
The diskette format microcode instructions from the DS microcontroller are listed on on this Web page. These were named by Dwight Elvey using the signals named on sheet #1 of the DS FDC-1. I've slightly edited the file Dwight sent me as he noted above. - Herb Johnson
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Copyright © 2008 Herb Johnson, from documents (C) Dwight Elvey