svi-328.dev

CP/M 2.27 for SV-608M — Disk Analysis

Analysis of stitched_cpm_227_608m_system_40ss.dsk, a clean CP/M 2.27 system disk for the Spectravideo SVI-328 with SV-608M memory/hard-disk expansion.

The disk image was reconstructed by stitching the best tracks from two independent dumps (dump5 and dump6) of the same physical floppy disk. The system tracks (0–1) are byte-identical across both dumps; only 4 bytes differed at track 16, sector 7, positions 252–255 (corrected from dump6).

Disk Format

Parameter	Value
Sides	1 (single-sided)
Tracks	40
Track 0	18 sectors × 128 bytes = 2,304 bytes
Tracks 1–39	17 sectors × 256 bytes = 4,352 bytes
Total size	172,032 bytes

System Layout

The CP/M system occupies tracks 0 and 1:

Component	Disk Location	Load Address	Size
Boot sector	Track 0, sector 1	C100h	128 bytes
BIOS	Track 0, sectors 2–18	D600h	5,248 bytes
	+ Track 1, sectors 1–12
CCP + BDOS	Track 1, sectors 13–17	C000h	5,632 bytes
	(+ next track)

Memory Map

0000h–00FFh   Zero page (BIOS vectors, system parameters)
C000h–C005h   CCP entry points (CCP_ENTRY, CCP_WBOOT_ENTRY)
C000h–C805h   CCP (Console Command Processor)
C806h–D5FFh   BDOS (Basic Disk Operating System)
D600h–D630h   BIOS jump table (17 entries)
DFB2h         BOOT_impl — cold boot
E098h         WBOOT_impl — warm boot
EEE7h–EF4Bh   DMA buffer area

Boot Sequence

1. SVI-328 BIOS loads track 0, sector 1 to C100h and jumps there
2. Boot sector configures PSG register 15 for memory banking (88h←0Fh, 8Ch←DDh)
3. Reads 17 sectors from track 0 (128-byte mode) into D600h
4. Switches FDC to 256-byte sector mode
5. Reads 12 sectors from track 1 into DE80h
6. Jumps to D600h (BIOS cold boot entry)
7. BIOS BOOT_impl (DFB2h): disables interrupts, initialises memory mapper, timer, RST 38h vector, and function keys
8. Sends reset to SV-608M SASI host adapter via OUT (40h), 00h (SUB39 at E227h — de-asserts all SASI bus signals)
9. Detects 80-column card (SUB09 at DC46h — probes port 50h/51h)
10. Initialises RS-232 UART (SUB10 at DCB1h — ports 28h–2Dh)
11. Prints banner: Spectravideo CP/M-80 Revision 2.27 For SV-608M Booted From Floppy Disk
12. Enters SUB32 (E09Bh): sets up zero-page vectors (0000h → JP D603h, 0005h → JP C806h)
13. Calls SUB40 (E233h) → SUB47 (E449h) to send the WD1002-SHD Set Parameters command via SASI. This is where the system hangs without SV-608M hardware (see below).
14. Reads CCP+BDOS (22 sectors from track 1, sector 13) into C000h
15. Patches BDOS hooks for serial I/O interception
16. Jumps to CCP at C000h

Hardware Identification

SASI Controller: Western Digital WD1002-SHD

The SV-608M communicates with its hard disk via the SASI (Shugart Associates System Interface) bus protocol. The SASI disk controller is a Western Digital WD1002-SHD Winchester Disk Controller board.

Reference: WD1002-SHD OEM Manual (Aug 1984)

The WD1002-SHD is an intelligent controller board containing a WD1010 Winchester controller chip, an 8X300 microprocessor for SASI protocol handling, and a built-in data separator with ECC support.

Hard Disk: Seagate ST-212

The attached hard disk is a Seagate ST-212, a 10 MB, 5.25” half-height Winchester drive.

Reference: Seagate ST-212 Product Manual (Rev. B)

Parameter	Value
Formatted capacity	10.0 MB
Cylinders	306
Heads	4
Tracks	1,224 (306 × 4)
Sectors per track	32
Bytes per sector	256
Bytes per track	8,192
Recording method	MFM
Data transfer rate	5.0 Mbit/s
Discs	2
Track-to-track seek	23 ms max
Average seek	65 ms max

BIOS vs physical capacity: The BIOS partitions use 1,160 of the 1,224 available tracks (290 of 306 cylinders), leaving 64 tracks (16 cylinders) at the inner edge unused — likely reserved for bad track alternates, which the WD1002-SHD supports via its Format Alternate Track command (opcode 0Eh).

SV-608M SASI Host Adapter

The SV-608M card provides a register-mapped interface to the raw SASI bus signals. The BIOS accesses two separate port groups:

SASI Bus Interface (ports 40h–46h)

Port	R/W	SASI Function
40h	W	Bus control (reset — writing 00h de-asserts all signals)
41h	W	Data bus output (host → bus), auto-generates ACK handshake
42h	R	Data bus input (bus → host), auto-generates ACK handshake
43h	W	SEL assertion (triggers Target Selection Phase)
44h	?	Additional control (not used in normal read/write)
46h	R	SASI bus status — reads 5 control signals as a 5-bit value

Status Register (port 46h) — SASI Bus Signal Mapping

The status register at port 46h directly exposes the 5 SASI bus control signals. The BIOS reads this register and masks with AND 1Fh to get the current bus phase:

Bit	Signal	Name
0	BSY	Busy
1	REQ	Request
2	C/D	Command/Data
3	I/O	Input/Output
4	MSG	Message

This maps directly to the Information Transfer Phase Configurations table from the WD1002-SHD manual (Table 6-1):

Status	Binary	BSY	REQ	C/D	I/O	MSG	SASI Phase	Direction
00h	00000	0	0	0	0	0	Bus Free	—
03h	00011	1	1	0	0	0	Data Out	Host → Controller
0Bh	01011	1	1	1	0	0	Command	Host → Controller
13h	10011	1	1	0	1	0	Data In	Controller → Host
1Bh	11011	1	1	1	1	0	Status	Controller → Host
1Fh	11111	1	1	1	1	1	Message In	Controller → Host

Every status value polled by the BIOS now has a precise meaning in the standard SASI bus protocol.

SASI Transaction Sequence

A complete SASI transaction, as performed by the BIOS, follows this sequence:

1. Bus Free (00h) — SUB53: Poll until (46h & 1Fh) == 00h
2. Target Selection — Write target address 01h to port 41h (data bus), then write 01h to port 43h (assert SEL). The WD1002-SHD responds by asserting BSY.
3. Command Phase (0Bh) — SUB50: Poll for 0Bh, then write each command byte to port 41h. The 6-byte SASI Group 0 command block is sent.
4. Data In Phase (13h) — SUB43: Poll for 13h, then INIR 256 bytes from port 42h. (For writes: Data Out Phase, 03h, OUTIR to port 41h.)
5. Status Phase (1Bh) — SUB51: Poll for 1Bh, read 1-byte status from port 42h. Bit 1 = error flag.
6. Message In Phase (1Fh) — SUB52: Poll for 1Fh, read 1-byte completion (always 00h, signals bus freed).

SV-608M Centronics Printer / Detection (ports 60h–63h)

Ports 60h–63h are a Centronics parallel printer interface on the SV-608M expansion unit, also used for hardware presence detection:

• SUB55 (E506h): Initialises the printer interface — clears strobe, aux and status ports, sets data port to FFh (idle), then writes 34h to aux and strobe
• SUB56 (E5A7h): Detects SV-608M presence — toggles bit 2 of AUX port (63h) and reads STATUS port (62h) before and after. If STATUS changes, the hardware is present (carry clear). If same, no hardware (carry set).

At boot, SUB59 (E8A6h) calls SUB55 and SUB56, then routes the CP/M LIST device:

• SV-608M present → LIST output via ports 60h–63h (active-low data, BIOS inverts character via CPL before writing to port 60h)
• SV-608M absent → LIST output via standard printer ports 10h–12h

Boot Impact

This BIOS unconditionally attempts to communicate with the SV-608M SASI adapter at boot time. The controller init has no timeout or fallback — SUB53 at E4DAh busy-waits forever for the Bus Free phase (port 46h & 1Fh == 0).

Without SV-608M hardware (or something that responds on port 46h), the system hangs immediately after printing the boot banner, stuck in the infinite poll loop at E4DAh.

Hard Disk Access Analysis

Drive Configuration

SELDSK_impl (DCECh) supports 5 drives (0–4, mapped to A:–E:). Each Disk Parameter Header (DPH) is 16 bytes starting at D739h. The DPH points to a Disk Parameter Block (DPB) that defines all geometry. All geometry is hardcoded in the DPBs — no runtime query of the disk’s physical geometry is performed.

Both the floppy BIOS and the HDD BIOS support only one floppy drive. The SVI-328 hardware can physically connect two floppy drives, but this CP/M 2.27 + 608M BIOS only provisions a single floppy in both builds.

Drive Maps

The floppy system disk and T0T1.SYS (on the HDD) contain different BIOS builds with different drive letter assignments. Both use the same 5 DPBs (D7A7h, D7C6h, D7D5h, D7E4h, D7F3h) but assign them to different drives:

Floppy BIOS (booted from floppy, A:=floppy)

Drive	DPH	DPB	SPT	BSH	Block	DSM	DRM	CKS	OFF	Capacity	Type
A:	D739h	D7A7h	34	4	2K	162	63	16	3	334K	Floppy (80-track)
B:	D749h	D7C6h	64	4	2K	1023	383	0	2	2,048K	HDD partition 1
C:	D759h	D7D5h	64	4	2K	1023	383	0	258	2,048K	HDD partition 2
D:	D769h	D7E4h	64	4	2K	1023	383	0	514	2,048K	HDD partition 3
E:	D779h	D7F3h†	64	4	2K	1559	511	0	770	3,120K	HDD partition 4

HDD BIOS (booted from HDD via T0T1.SYS, A:=HDD)

Drive	DPH	DPB	SPT	BSH	Block	DSM	DRM	CKS	OFF	Capacity	Type
A:	D739h	D7C6h	64	4	2K	1023	383	0	2	2,048K	HDD partition 1
B:	D749h	D7D5h	64	4	2K	1023	383	0	258	2,048K	HDD partition 2
C:	D759h	D7E4h	64	4	2K	1023	383	0	514	2,048K	HDD partition 3
D:	D769h	D7F3h	64	4	2K	1559	511	0	770	3,120K	HDD partition 4
E:	D779h	D7A7h	34	4	2K	162	63	16	3	326K	Floppy (80-track)

The DPB pointers rotate: the floppy BIOS puts the floppy DPB (D7A7h) on drive A: and the four HDD DPBs on B:–E:; the HDD BIOS shifts the HDD DPBs to A:–D: and puts the floppy DPB on E:. This means files on HDD partition 1 appear as B: when booted from floppy, but as A: when booted from HDD.

Column definitions (standard CP/M 2.2 Disk Parameter Block fields):

• DPH = Disk Parameter Header address in memory (16-byte structure containing pointers to the translation table, directory buffer, DPB, and allocation vectors).
• DPB = Disk Parameter Block address (the structure containing the geometry fields below).
• SPT = Sectors Per Track — number of logical 128-byte sectors per track. SPT=64 → 32 physical 256-byte sectors (8,192 bytes/track). SPT=34 → 17 physical 256-byte sectors (4,352 bytes/track).
• BSH = Block Shift — log₂(block size / 128). BSH=4 → 2K blocks, BSH=3 → 1K blocks.
• Block = allocation block size (derived from BSH: 128 × 2^BSH).
• DSM = maximum Data block (Storage) number (0-based). Total data blocks = DSM + 1. Capacity = (DSM + 1) × block size.
• DRM = maximum Directory entry number (0-based). Total directory entries = DRM + 1. Each entry is 32 bytes.
• CKS = Checksum vector size. CKS=0 indicates non-removable media (hard disk); CKS>0 indicates removable media (floppy) — CP/M checksums the first CKS directory entries to detect disk changes.
• OFF = track Offset — number of reserved/system tracks before the data area begins.
• † Drive E’s DPB pointer can be switched at runtime by the routine at D7B6h, which toggles between two alternate floppy DPBs (see below).

Floppy DPB Variants

Two additional DPBs exist for the floppy drive to support 40-track floppy formats. The subroutine at D7B6h toggles the DPB pointer in DPH D779h (drive E: in both builds). In the HDD BIOS, drive E: is the floppy, so this toggle selects between 40-track formats. In the floppy BIOS, drive E: defaults to HDD partition 4; the toggle’s intended use in that context is unclear. The subroutine at D7B6h switches this DPH to point to one of these:

DPB	SPT	Block	DSM	Capacity	Format
D789h	34	1K	156	157K	40-track SS, 17×256-byte
D798h	32	1K	131	132K	40-track SS, 16×256-byte

The toggle at D7B6h reads the current DPB pointer from the DPH, and if it points to D789h, switches to D798h, and vice versa.

80-Track Floppy DPB

The floppy DPB (D7A7h) has SPT=34 and DSM=162 with 2K blocks. The resulting capacity (163 × 2,048 = 333,824 bytes) requires 77 data tracks plus 3 system tracks (OFF=3) = 80 tracks total. This is consistent with an 80-track single-sided floppy drive, distinct from the standard SVI-328 40-track drive. This DPB is used for drive A: in the floppy BIOS, or drive E: in the HDD BIOS.

HDD Partition Layout

The four HDD partitions are laid out sequentially on the same hard disk, addressed by a flat track numbering scheme:

Track    0     1     2        257   258       513   514       769   770       1159
         ├─────┤     ├─────────┤     ├─────────┤     ├─────────┤     ├─────────┤
         reserved     Part 1, 2MB     Part 2, 2MB     Part 3, 2MB     Part 4, ~3MB
         (2 trks)    (256 trks)      (256 trks)      (256 trks)      (390 trks)

Drive letter assignment depends on which BIOS is running:

• Floppy BIOS: Partition 1–4 = B:, C:, D:, E:
• HDD BIOS: Partition 1–4 = A:, B:, C:, D:

Total HDD capacity: 1,160 tracks × 8,192 bytes/track = 9,502,720 bytes (~9.05 MB) across 4 partitions.

Each HDD track contains 32 physical sectors of 256 bytes (= 64 CP/M logical 128-byte sectors), yielding 8,192 bytes per track.

Physical Access Protocol (via SASI)

The hard disk is accessed through the SV-608M SASI host adapter (ports 40h–46h). All commands follow the standard SASI bus phase sequence described above.

Linear Sector Addressing

SUB36 (E15Bh, read) and SUB37 (E1B1h, write) convert the CP/M track/sector to a 16-bit linear sector address:

linear_sector = track × 32 + sector − 1

This is computed using a multiply loop (ADD IX,DE repeated 32 times at E18Ah/E1E0h), where DE = track number from SETTRK, and the sector from SETSEC is added at the end.

Read Command (SUB41 → SUB42, SUB43)

SUB42 (E3A9h) builds a 6-byte SASI Group 0 command block at E4F9h:

Offset	Value	SASI Field
0	08h	Command class 0, opcode 08h (Read)
1	00h	LUN=0 (bits 7-5), sector addr bits 20-16=0 (bits 4-0)
2	sector addr high	Logical sector address, bits 15-8
3	sector addr low	Logical sector address, bits 7-0
4	01h	Block count = 1 sector
5	47h	WD1002-SHD control byte (see below)

Control byte 47h (01000111b) decoded per WD1002-SHD Table 4-2:

Bits	Field	Value	Meaning
7	R	0	Retries enabled (up to 10 + restore)
6	A	1	Immediate ECC correction enabled
5	—	0	Reserved
4	—	0	Reserved
3-0	STEP	0111	Fast step option 7: 15 µs per step

SUB49 (E48Eh) sends these 6 bytes to port 41h during the Command Phase (status 0Bh). SUB43 (E3D0h) then reads 256 bytes from port 42h during the Data In Phase (status 13h) using INIR with B=0.

Write Command (SUB44 → SUB45, SUB46)

SUB45 (E404h) builds the same 6-byte SASI command block, but with opcode 0Ah (Write) and control byte 07h (retries enabled, no ECC, step option 7).

SUB46 (E42Bh) waits for the Data Out Phase (status 03h) and writes 256 bytes to port 41h using OUTIR with B=0.

Controller Initialisation — Set Parameters (SUB47/SUB48)

During boot, SUB47 (E449h) sends the WD1002-SHD Set Parameters command (SASI opcode 0Ch, aka “Initialize Drive Characteristics”) followed by an 8-byte parameter block sent via SUB48 (E463h) during the Data Out Phase:

Byte	Value	WD1002-SHD Parameter	Decoded
0	01h	MSByte of Number of Cylinders	0x0132 = 306
1	32h	LSByte of Number of Cylinders
2	04h	Number of Heads	4 heads
3	00h	MSByte of Reduced Write Current Cylinder	0x0080 = 128
4	80h	LSByte of Reduced Write Current Cylinder
5	00h	MSByte of Write Precomp Cylinder	0x0040 = 64
6	40h	LSByte of Write Precomp Cylinder
7	0Bh	Max ECC Error Burst Length	11 bits

These values precisely match the Seagate ST-212’s physical geometry (306 cylinders, 4 heads). The WD1002-SHD power-up defaults are for the ST-506 (153 cylinders, 4 heads, RWC=128, WP=64, ECC=11) — the BIOS only needs to change the cylinder count from 153 to 306; all other parameters happen to be the same between the ST-506 and ST-212.

SV-608M Detection (SUB55, SUB56)

The BIOS detects SV-608M presence using its Centronics printer ports 60h–63h (not the SASI ports):

1. SUB55 (E506h): Initialises the printer interface — 00h to strobe (61h), aux (63h) and status (62h), FFh to data (60h), then 34h to aux (63h) and strobe (61h).

2. SUB56 (E5A7h): Toggles bit 2 of AUX port (63h) and reads STATUS port (62h) before and after. If STATUS changes, the SV-608M is present (carry clear). If same, no hardware (carry set).

This detection runs in SUB59 (E8A6h) during cold boot. The result determines both the LIST device routing (SV-608M printer vs standard printer) and whether DATA69 (E89Fh) is set to 0 (present) or 1 (absent).

Key Observations

• Standard SASI bus protocol. The entire SV-608M I/O protocol is textbook SASI (Shugart Associates System Interface). Port 46h is a direct readout of the 5 SASI control signals (BSY, REQ, C/D, I/O, MSG), and the 6-byte command blocks are standard SASI Group 0 format. Ports 41h/42h are the bidirectional SASI data bus with hardware-generated ACK handshake.

• WD1002-SHD command set. The BIOS uses three WD1002-SHD commands: Set Parameters (0Ch) at boot, Read (08h), and Write (0Ah). The control byte configures ECC correction, retry policy, and step rate per-command.

• All disk geometry is statically defined in the DPBs at compile time. The BIOS does not query the hard disk for its geometry — the ST-212’s 306 cylinders and 4 heads are hardcoded in the Set Parameters block.

• Sector size is always 256 bytes for both floppy and HDD. The CP/M 128-byte logical sectors are paired internally by SETSEC_impl (DD13h), which halves the sector number and tracks odd/even in DATA24 (E149h).

• The WD1002-SHD handles all CHS translation. The BIOS sends a 16-bit linear sector address. The WD1002-SHD maps this to cylinder/head/sector internally using the parameters set by the 0Ch command. The Z80 never deals with physical CHS geometry.

• 64 tracks (16 cylinders) reserved. The ST-212 has 1,224 tracks but the BIOS only partitions 1,160. The unused 64 tracks at the inner edge are likely reserved for the WD1002-SHD’s bad track alternates.

• No sector translation table is used (all DPH XLT pointers are 0000h). SECTRAN_impl (DCE8h) simply adds 1 to the sector number (making hardware sectors 1-based).

Files

CP/M System Disassemblies

Extracted from images/stitched_cpm_227_608m_system_40ss.dsk. All disassemblies were generated with z80dismblr and post-processed to replace raw port numbers with named EQU constants (see svi328-port-map.md for the full port reference).

File	Origin	Lines	Description
boot_sector.asm	C100h	151	Boot sector with FDC/PSG port EQUs
cpm_system.asm	D600h	5,281	BIOS with full port EQUs (RS-232, FDC, SV608M, 80-col, HDD, PSG)
ccp_bdos.asm	C000h	5,539	CCP + BDOS (no direct port I/O)

SVI-609 Bootstrap Card

Disassembly and analysis of the SVI-609 Bootstrap Card ROM (images/svi-609.bin, 4096 bytes).

File	Description
svi-609.asm	Annotated disassembly (bootstrap stub at 0000h + main code at 8000h)
svi-609-analysis.md	Full analysis: boot mechanism, SASI/FDC protocol, comparison with CP/M BIOS

Documentation

File	Description	Based on
svi328-port-map.md	SVI-328 I/O port reference (FDC, SASI, PSG, 80-col, etc.)	BIOS disassembly + WD1002-SHD manual
hdd-dump-program.md	HDD dump methodology and SASI protocol reference	BIOS SASI routines + WD1002-SHD manual + ST-212 specs
hdd-dump.asm	HDD dump utility — standalone ROM (org 0000h, VDP text output)	BIOS SASI routines + WD1002-SHD manual + ST-212 specs
svi-609-analysis.md	SVI-609 Bootstrap Card ROM analysis	svi-609.asm disassembly
msysgen.md	MSYSGEN.COM analysis — CP/M system generation (floppy ↔ hard disk)	MSYSGEN.asm disassembly
mformat.md	MFORMAT.COM analysis — hard disk and floppy disk formatter	MFORMAT.asm disassembly

Reference PDFs

File	Description
79-000004_WD1002-SHD_OEM_Manual_Aug1984.pdf	Western Digital WD1002-SHD Winchester Disk Controller OEM Manual
Seagate-ST212-Hard-Disk.pdf	Seagate ST-212 Hard Disk Product Manual (Rev. B)

Binary Images

File	Size	Description
stitched_cpm_227_608m_system_40ss.dsk	172,032 bytes	Clean CP/M 2.27 floppy disk image
svi-609.bin	4,096 bytes	SVI-609 Bootstrap Card ROM dump
boot_sector.bin	128 bytes	Track 0, sector 1
cpm_system.bin	5,248 bytes	BIOS (track 0 sec 2–18 + track 1 sec 1–12)
ccp_bdos.bin	5,632 bytes	CCP + BDOS
track0_sec2_18.bin	2,176 bytes	Intermediate: track 0 sectors 2–18
track1_sec1_12.bin	3,072 bytes	Intermediate: track 1 sectors 1–12

Utilities

File	Description
add_port_equs.py	Script to add port EQUs to disassemblies

Port Groups Used

The BIOS accesses the following I/O port groups (functional roles inferred from code behaviour):

• RS-232 UART — ports 28h–2Dh (serial I/O)
• Floppy disk controller — ports 30h–38h (FDC commands, sector R/W)
• SV-608M SASI host adapter — ports 40h–46h (SASI bus interface to WD1002-SHD)
• 80-column card — ports 50h–58h (CRTC registers, bank switching)
• SV-608M Centronics printer — ports 60h–63h (parallel printer + presence detection)
• PSG — ports 88h/8Ch/90h (register 15 used for memory bank switching)
• Port DCh — unknown function (used in LIST output routine)