PC-8801 Disk subsystem

■ Disk subsystem

■ I/O ports of disk subsystem I/F

Address

R/W

Description

FCh

R/W

Disk subsystem I/F 8255 port A
In the standard state, it is a data reception port from the subsystem.

FDh

R/W

Disk subsystem I/F 8255 port B
In the standard state, it is a data transmission port to the subsystem.

FEh

R/W

Disk subsystem I/F 8255 port C

Bit	7	6	5	4	3	2	1	0
R/W	W				R
Signal	ATN	DAC	RFD	DAV		DAC	RFD	DAV

ATN	W	Attention	Set to 1 when sending a command to the subsystem or interrupting a command
DAC		Data Accepted	Set to 1 after fetching data from the subsystem
RFD		Ready For Data	Set to 1 when requesting data from the subsystem
DAV		Data Valid	Set to 1 after outputting data to port B
DAC	R	Data Accepted	Becomes 1 after the subsystem picks up data
RFD		Ready For Data	Becomes 1 when the subsystem requests data
DAV		Data Valid	Becomes 1 after data is output to port A

FFh

Disk subsystem I/F 8255 control

■ Communication protocols with the subsystem

Send command:
1. Transmitter sets ATN = 1
2. Receiver detects ATN == 1 and sets RFD = 1
3. Transmitter detects RFD == 1 and sets ATN = 0
4. Transmitter puts the data on the bus
5. Transmitter sets DAV = 1
6. Receiver detects DAV == 1 and sets RFD = 0
7. Receiver receives data
8. Receiver sets DAC = 1
9. Transmitter detects DAC == 1 and sets DAV = 0
10. Receiver detects DAV == 0 and sets DAC = 0

Normal handshake:
1. Receiver sets RFD = 1
2. Transmitter detects RFD == 1 and puts data on the bus
3. Transmitter sets DAV = 1
4. Receiver detects DAV == 1 and sets RFD = 0
5. Receiver receives data
6. Receiver sets DAC = 1
7. Transmitter detects DAC == 1 and sets DAV = 0
8. Receiver detects DAV == 0 and sets DAC = 0

Fast handshake:
1. Receiver sets RFD = 1
2. Transmitter detects RFD == 1 and puts data (n) on the bus
3. Transmitter sets DAV = 1
4. Receiver detects DAV == 1 and sets RFD = 0
5. Receiver receives data
6. Receiver sets DAC = 1
7. Transmitter detects DAC == 1 and puts data (n+1) on the bus
8. Transmitter sets DAV = 0
9. Receiver detects DAV == 0 and receives data
10. Receiver sets DAC = 0

■ Routines in ROM that communicate with the subsystem

Label	Address		Function
Label	N88	N	Function
PUTCOM	37CCh	017Ch	Send the contents of the A register as a command to the subsystem
PUTDAT	37D2h	0183h	Send the contents of the A register to the subsystem
GETDAT	3847h	01E9h	Return the data received from the subsystem to the A register

*N88: If the work area (EF15h) is not 00h, high-speed handshake is performed using the contents of the L register as the 2nd byte.

■ Subsystem commands

The command number is sent by command transmission, and the parameters are usually sent by handshake.

<Drive number>	0..1
<Track number>	0..79
<Sector number>	1..16 (2D)

Command number 00h

Function name Initialize

Command format Host → Sub: <00h>

Function Turn on the drive motor, reset the FDC, return the head to the initial position,
and do not change the drive operation mode (duplex/single-sided) and read-after-write check mode.

Command number	00h
Function name	Initialize
Command format	Host → Sub: <00h>
Function	Turn on the drive motor, reset the FDC, return the head to the initial position, and do not change the drive operation mode (duplex/single-sided) and read-after-write check mode.

Command number 01h

Function name Write Data

Command format Host → Sub: <01h> <Number of sectors> <Drive number> <Track number> <Sector number> <Data stream...>

Function Write data sent from the host to disk.

Command number	01h
Function name	Write Data
Command format	Host → Sub: <01h> <Number of sectors> <Drive number> <Track number> <Sector number> <Data stream...>
Function	Write data sent from the host to disk.

Command number 02h

Function name Read Data

Command format Host → Sub: <02h> <Number of sectors> <Drive number> <Track number> <Sector number>

Function Read data from the specified sector and stores it in the subsystem's buffer.
To obtain the data, the host must execute the Send Data command in succession.

Command number	02h
Function name	Read Data
Command format	Host → Sub: <02h> <Number of sectors> <Drive number> <Track number> <Sector number>
Function	Read data from the specified sector and stores it in the subsystem's buffer. To obtain the data, the host must execute the Send Data command in succession.

Command number 03h

Function name Send Data

Command format Host → Sub: <03h>
Sub → Host: <Data stream...>

Function Transfer the data of the size read by the previous Read Data command to the host.

Command number	03h
Function name	Send Data
Command format	Host → Sub: <03h> Sub → Host: <Data stream...>
Function	Transfer the data of the size read by the previous Read Data command to the host.

Command number 04h

Function name Copy

Command format Host → Sub: <04h> <Number of sectors> <Drive number 1> <Track number 1> <Sector number 1> <Drive number 2> <Track number 2> <Sector number 2>

Function Copy sector contents sector by sector.

Command number	04h
Function name	Copy
Command format	Host → Sub: <04h> <Number of sectors> <Drive number 1> <Track number 1> <Sector number 1> <Drive number 2> <Track number 2> <Sector number 2>
Function	Copy sector contents sector by sector.

Command number 05h

Function name Format

Command format Host → Sub: <05h> <Drive number>

Function Format the disk.

Command number	05h
Function name	Format
Command format	Host → Sub: <05h> <Drive number>
Function	Format the disk.

Command number 06h

Function name Send Result Status

Command format Host → Sub: <06h>
Sub → Host: <Status>

Function The execution result of the command sent just before is transmitted to the main side.

If the 0th bit of the result status is 1, there is an error.
If the 6th bit is 1, there is data in the read buffer.
If the 7th bit is 1, the I/O operation is finished.

Command number	06h
Function name	Send Result Status
Command format	Host → Sub: <06h> Sub → Host: <Status>
Function	The execution result of the command sent just before is transmitted to the main side. If the 0th bit of the result status is 1, there is an error. If the 6th bit is 1, there is data in the read buffer. If the 7th bit is 1, the I/O operation is finished.

Command number 07h

Function name Send Drive Status

Command format Host → Sub: <07h>
Sub → Host: <Status>

Function Bits 7 to 4 of the drive status indicate the connection status of the drive.

Command number	07h
Function name	Send Drive Status
Command format	Host → Sub: <07h> Sub → Host: <Status>
Function	Bits 7 to 4 of the drive status indicate the connection status of the drive.

Command number 08h

Function name Test Memory

Command format Host → Sub: <08h>
Sub → Host: <Status>

Function Perform subsystem memory tests:
80h: OK
81h: Not good

Command number	08h
Function name	Test Memory
Command format	Host → Sub: <08h> Sub → Host: <Status>
Function	Perform subsystem memory tests: 80h: OK 81h: Not good

Command number 09h

Function name Send Memory

Command format Host → Sub: <09h> <Start address H> <Start address L> <End address H> <End address L>
Sub → Host: <Data stream...>

Function Transfer subsystem memory content to host.

Command number	09h
Function name	Send Memory
Command format	Host → Sub: <09h> <Start address H> <Start address L> <End address H> <End address L> Sub → Host: <Data stream...>
Function	Transfer subsystem memory content to host.

Command number 0Ah

Function name Outmargin Support

Command format Host → Sub: <0Ah> <Data>

Function Output specified data to the subsystem I/O port F7h (no use value).

Command number	0Ah
Function name	Outmargin Support
Command format	Host → Sub: <0Ah> <Data>
Function	Output specified data to the subsystem I/O port F7h (no use value).

Command number 0Bh

Function name Send Memory 2

Command format Host → Sub: <0Bh> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L>
Sub → Host: <Data stream...>

Function Transfer subsystem memory content to host.

Command number	0Bh
Function name	Send Memory 2
Command format	Host → Sub: <0Bh> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L> Sub → Host: <Data stream...>
Function	Transfer subsystem memory content to host.

Command number 0Ch

Function name Receive Memory

Command format Host → Sub: <0Ch> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L>

Function Transfer data from host to the subsystem memory.

Command number	0Ch
Function name	Receive Memory
Command format	Host → Sub: <0Ch> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L>
Function	Transfer data from host to the subsystem memory.

Command number 0Dh

Function name Execute

Command format Host → Sub: <0Dh> <Start address H> <Start address L>

Function Start execution in the subsystem from a specified address.

Command number	0Dh
Function name	Execute
Command format	Host → Sub: <0Dh> <Start address H> <Start address L>
Function	Start execution in the subsystem from a specified address.

Command number 0Eh

Function name Load Data

Command format Host → Sub: <0Eh> <Number of sectors> <Drive number> <Track number> <Sector number> <Start address H> <Start address L>

Function Read data from disk and store it at a specified address in the subsystem.

Command number	0Eh
Function name	Load Data
Command format	Host → Sub: <0Eh> <Number of sectors> <Drive number> <Track number> <Sector number> <Start address H> <Start address L>
Function	Read data from disk and store it at a specified address in the subsystem.

Command number 0Fh

Function name Save Data

Command format Host → Sub: <0Fh> <Number of sectors> <Drive number> <Track number> <Sector number> <Start address H> <Start address L>

Function Writes data to disk from a specified address in the subsystem.

Command number	0Fh
Function name	Save Data
Command format	Host → Sub: <0Fh> <Number of sectors> <Drive number> <Track number> <Sector number> <Start address H> <Start address L>
Function	Writes data to disk from a specified address in the subsystem.

Command number 10h

Function name Load & Go

Command format Host → Sub: <10h>

Function After initialization, load 256 bytes of data from drive 1, track 0, sector 1 into 5000h in the subsystem memory and execute it.

Command number	10h
Function name	Load & Go
Command format	Host → Sub: <10h>
Function	After initialization, load 256 bytes of data from drive 1, track 0, sector 1 into 5000h in the subsystem memory and execute it.

Command number 11h

Function name High Speed Write Data

Command format Host → Sub: <11h> <Number of sectors> <Drive number> <Track number> <Sector number> <Data stream...>

Function Same as 01h write data except that the data stream is transferred using high-speed handshake.

Command number	11h
Function name	High Speed Write Data
Command format	Host → Sub: <11h> <Number of sectors> <Drive number> <Track number> <Sector number> <Data stream...>
Function	Same as 01h write data except that the data stream is transferred using high-speed handshake.

Command number 12h

Function name High Speed Send Data

Command format Host → Sub: <12h>
Sub → Host: <Data stream...>

Function Same as 03h send data except that the data stream is transferred using high-speed handshake.

Command number	12h
Function name	High Speed Send Data
Command format	Host → Sub: <12h> Sub → Host: <Data stream...>
Function	Same as 03h send data except that the data stream is transferred using high-speed handshake.

Command number 13h

Function name Error Info

Command format Host → Sub: <13h>
Sub → Host: <Command> <ST0> <ST1> <ST2> <C> <H> <R> <N>

Function Transfer the last command executed by the FDC and the FDC status to the host.

Command number	13h
Function name	Error Info
Command format	Host → Sub: <13h> Sub → Host: <Command> <ST0> <ST1> <ST2> <C> <H> <R> <N>
Function	Transfer the last command executed by the FDC and the FDC status to the host.

Command number 14h

Function name Sense Device Status

Command format Host → Sub: <14h> <Drive number>
Sub → Host: <Status>

Function Returns FDC status to host side

Bit 0 Unit Select 0

Bit 1 Unit Select 1

Bit 2 Head Selection

Bit 3 Two Sided

Bit 4 Track 00

Bit 5 Ready (always 1)

Bit 6 Write Protected

Bit 7 Fault

Command number 15h

Function name Fast Send Memory

Command format Host → Sub: <15h> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L>
Sub → Host: <Data stream...>

Function Same as 0Bh send memory except when fast handshaking is used to transfer data.
The number of transferred bytes must be an even number.

Command number	15h
Function name	Fast Send Memory
Command format	Host → Sub: <15h> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L> Sub → Host: <Data stream...>
Function	Same as 0Bh send memory except when fast handshaking is used to transfer data. The number of transferred bytes must be an even number.

Command number 16h

Function name High Speed Receive Memory

Command format Host → Sub: <16h> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L> <Data string...>

Function Same as 0Ch receive memory except when fast handshaking is used to transfer data.
The number of transferred bytes must be an even number.

Command number	16h
Function name	High Speed Receive Memory
Command format	Host → Sub: <16h> <Start address H> <Start address L> <Number of bytes H> <Number of bytes L> <Data string...>
Function	Same as 0Ch receive memory except when fast handshaking is used to transfer data. The number of transferred bytes must be an even number.

Command number 17h

Function name Set Surface Mode

Command format Host → Sub: <17h> <Mode H>

Function Specify whether each drive should be in simplex or duplex mode

Bit 3 to Bit 0: 0: Single-sided 1: Double-sided

At startup, all drives are in single-sided mode.

Command number	17h
Function name	Set Surface Mode
Command format	Host → Sub: <17h> <Mode H>
Function	Specify whether each drive should be in simplex or duplex mode Bit 3 to Bit 0: 0: Single-sided 1: Double-sided At startup, all drives are in single-sided mode.

Command number 18h

Function name Send Surface Mode

Command format Host → Sub: <18h>
Sub → Host: <Surface mode>

Function Send surface mode to host.

Command number	18h
Function name	Send Surface Mode
Command format	Host → Sub: <18h> Sub → Host: <Surface mode>
Function	Send surface mode to host.

Command number 19h

Function name Set Read After Write

Command format Host → Sub: <19h>

Function Perform a read-after-write check after executing this command,
a read-after-write check will be automatically performed after executing a command that involves writing to the disk.

If an error is detected, the error bit in the result status will be set.

Command number	19h
Function name	Set Read After Write
Command format	Host → Sub: <19h>
Function	Perform a read-after-write check after executing this command, a read-after-write check will be automatically performed after executing a command that involves writing to the disk. If an error is detected, the error bit in the result status will be set.

Command number 1Ah

Function name Reset Read After Write

Command format Host → Sub: <1Ah>

Function Disable read-after-write checks.

Command number	1Ah
Function name	Reset Read After Write
Command format	Host → Sub: <1Ah>
Function	Disable read-after-write checks.

Command number 1Bh

Function name Resume

Command format Host → Sub: <1Bh>

Function Resume execution of the subsystem stopped at a previously set breakpoint.
It is also possible to change the PC using the set register command and execute from that address.

Command number	1Bh
Function name	Resume
Command format	Host → Sub: <1Bh>
Function	Resume execution of the subsystem stopped at a previously set breakpoint. It is also possible to change the PC using the set register command and execute from that address.

Command number 1Ch

Function name Set Breakpoint

Command format Host → Sub: <1Ch> <Address H> <Address L>

Function Set a breakpoint at the specified address.
Breakpoints can only be set at one location.

Reset when stopped at breakpoint.

Command number	1Ch
Function name	Set Breakpoint
Command format	Host → Sub: <1Ch> <Address H> <Address L>
Function	Set a breakpoint at the specified address. Breakpoints can only be set at one location. Reset when stopped at breakpoint.

Command number 1Dh

Function name Set Register

Command format Host → Sub: <1Dh> <Register ID number> <Data H> <Data L>

Function Specify a register content when restarting execution with the resume command (1Bh).

Register ID Register

0 AF

1 BC

2 DE

3 HL

4 AF'

5 BC'

6 DE'

7 HL'

8 IX

9 IY

10 - I

11 PC

12 SP

Command number 1Eh

Function name Send Register

Command format Host → Sub: <1Eh> <Register ID number>
Sub → Host: <Data H> <Data L>

Function Send a register content to the host when the subsystem is stopped at the breakpoint.

Command number	1Eh
Function name	Send Register
Command format	Host → Sub: <1Eh> <Register ID number> Sub → Host: <Data H> <Data L>
Function	Send a register content to the host when the subsystem is stopped at the breakpoint.

■ Extended subsystem commands on machines equipped with 2HD drives

Command number 1Fh

Function name Set Disk Mode

Command format Host → Sub: <1Fh> <Drive number> <Disc code>

Function Set disc mode according to specified disc code.

Disc code:

Bit 7 6 5 4 3 2 1 0

Signal 0 0 DSK SPC 0 SCR

DSK Disc type 00: 1D/2D type
01: 1DD/2DD type
10: 1HD/2HD type

SPC 1HDs/2HDs selection 0: 1HD
1: 2HD

SCR Sectors length 01: 256 bytes/sector
10: 512 bytes/sector
11: 1024 bytes/sector

Command number 20h

Function name Send Disk Mode

Command format Host → Sub: <20h> <Drive number>
Sub → Host: <Disc code>

Function Send disk mode to host.

Command number	20h
Function name	Send Disk Mode
Command format	Host → Sub: <20h> <Drive number> Sub → Host: <Disc code>
Function	Send disk mode to host.

Command number 21h

Function name Set Boundary Mode

Command format Host → Sub: <21h> <Boundary Mode>

Function Set boundary mode.

0: Limit read/write range to 1D/2D disks
1: Limit the read/write range to disks with a sector length of 256 bytes (including disks with 1HDs/2HDs)
2: Allow read/write of all disks that can be supported by the subsystem

Command number	21h
Function name	Set Boundary Mode
Command format	Host → Sub: <21h> <Boundary Mode>
Function	Set boundary mode. 0: Limit read/write range to 1D/2D disks 1: Limit the read/write range to disks with a sector length of 256 bytes (including disks with 1HDs/2HDs) 2: Allow read/write of all disks that can be supported by the subsystem

Command number 22h

Function name Send Boundary Mode

Command format Host → Sub: <22h>
Sub → Host: <Boundary mode>

Function Send boundary mode to host.

Command number	22h
Function name	Send Boundary Mode
Command format	Host → Sub: <22h> Sub → Host: <Boundary mode>
Function	Send boundary mode to host.

Command number 23h

Function name Drive Ready Check

Command format Host → Sub: <23h> <Drive number>
Sub → Host: <Status>

Function Disk insertion check.

00h: Disc inserted
FFh: No disc inserted

Command number	23h
Function name	Drive Ready Check
Command format	Host → Sub: <23h> <Drive number> Sub → Host: <Status>
Function	Disk insertion check. 00h: Disc inserted FFh: No disc inserted

Command number 24h

Function name Expanded Format

Command format Host → Sub: <24h> <Drive number> <Data>

Function Format the disk with initialization data.

Command number	24h
Function name	Expanded Format
Command format	Host → Sub: <24h> <Drive number> <Data>
Function	Format the disk with initialization data.

■ About 1HDs/2HDs

To be autobootable 1HD/2HD discs must have track 0 formatted in FM mode with a sector length of 128 bytes and the rest in MFM mode with a sector length of 256 bytes.

Track 0 is read/written in units of 256 bytes, so only odd sector numbers can be specified.
The number of sectors should be 1/2 of the actual number.

■ I/O port on the subsystem side

Address

R/W

Description

F4h

Drive control

Bit	7	6	5	4	3	2	1	0
Signal			CLK	DSI	TD1	TD0	RV1	RV0

CLK	?	?
DSI	?	?
TD1	Drive 2 track density	0: 48TPI (2D) 1: 96TPI (2DD/2HD)
TD0	Drive 1 track density	0: 48TPI (2D) 1: 96TPI (2DD/2HD)
RV1	Drive 2 mode	0: 2D/2DD 1: 2HD
RV0	Drive 1 mode	0: 2D/2DD 1: 2HD

F8h

Motor control

Bit	7	6	5	4	3	2	1	0
Signal					PSEN		MON1	MON0

PSEN	?	?
MON1	Drive 2 motor control	0: Motor OFF 1: Motor ON
MON0	Drive 1 motor control	0: Motor OFF 1: Motor ON

FDC μPD765A TC input
When this port is read, the TC signal is input to FDC.

FAh

FDC μPD765A status register read

Bit	7	6	5	4	3	2	1	0
Signal	RQM	DIO	NDM	CB	D3B	D2B	D1B	D0B

RQM	Request for Master	When DIO = 0: Data can written to the data register. When DIO = 1: Data can be read from the data register
DIO	Data Input/Output	0: CPU → FDC 1: CPU ← FDC
NDM	Non-DMA Mode	Transferring data in Non-DMA mode
CB	FDC Busy	1: FDC is executing the command phase, result phase, and execution phase. Commands cannot be accepted.
DnB	FDn Busy	Drive n is executing a seek operation using the SEEK command, or an interrupt request to complete the seek operation is pending.

FBh

FDC μPD765A data register write

FDC μPD765A data register read

FCh

R/W

Disk subsystem I/F 8255 port A
In the standard state, it is a data reception port from the host.

FDh

R/W

Disk subsystem I/F 8255 port B
In the standard state, it is a data transmission port to the host.

FEh

R/W

Disk subsystem I/F 8255 port C

Bit	7	6	5	4	3	2	1	0
R/W	W				R
Signal		DAC	RFD	DAV	ATN	DAC	RFD	DAV

FFh

Disk subsystem I/F 8255 control

■ Subsystem memory map

    0000h   +---------------+
            |  System ROM   |
    0800h   |---------------|
            |    Vacant     |
    4000h   |---------------|
            |  Write Buffer |
    5000h   |---------------|
            |  Read Buffer  |
    6000h   |---------------|
            |    Unused     |
    7F00h   |---------------|
            |   Work Area   |
    8000h   |---------------|
            |    Vacant     |
    FFFFh   +---------------+

The M series system ROM is 8K bytes (0000h to 1FFFh).