Amstrad Extended BIOS Internals

Any Amstrad CP/M Plus system (meaning those created by Locomotive Software for the CPC, PCW and Spectrum +3) has an XBIOS, which lives at 80h in CP/M bank 0. It exposes a number of functions for things like reading, writing and formatting floppy discs, reassigning keys on the keyboard, setting options for the serial port, and so on.

In a conventional banked CP/M Plus system, bank 0 holds only the banked BDOS, a BIOS to manage the hardware, and possibly some disc buffers. In an Amstrad/Locomotive CP/M Plus system, the banked BIOS is small and pretty much hardware-independent; all it does is wrap the XBIOS calls in CP/M character and disc devices.

Internally, the XBIOS is divided into modules (this is why some functions in the jumpblock have names starting DD, some starting CD, some starting TE and so on). I have given these modules names based on the functions in them, on other sources where available, and invented my own names if necessary. When disassembling, it's often quite easy to tell where one module stops and another starts (at least in early versions) because the modules are aligned on 8- or 16- byte boundaries.

Sources

I have derived the information in this document from the following CP/M boot files:

If you have any other versions of the system file, feel free to share them with me.

The CPC

On the CPC, CP/M Plus runs as an application under the CPC's ROM firmware. This is the memory layout of bank 0 (with areas owned by the XBIOS shown thus):

The PCW (Old World)

The original versions of CP/M supplied with the PCW8256 (versions 1.2, 1.4) have a similar layout to the CPC version (give or take the constraints of hardware, such as the position and size of video RAM). However, the PCW doesn't have a firmware ROM, so it has to provide modules of its own to do the same job.

The addresses in this table come from BIOS 1.2. Those in the more common 1.4 are slightly different, but the arrangement of modules is the same.

The process and semaphore features in the kernel are used for dot-matrix printer support. Semaphores are used to block disc access while the printer is active (or vice versa), while processes manage different aspects of the printer. In a PCW8256 CP/M system, four processes will be in existence, at different priority levels:

Priority 32: CP/M
Priority 48: Printer control state
Priority 56: Screen dump
Priority 60: Printer hardware driver

Normally, the three printer processes will be blocked on semaphores. If they are invoked (eg, by the right key combination, or by sending output to the printer) the appropriate semaphore will be raised. The corresponding process will then have a higher priority than CP/M, and so will run until it next becomes blocked.

The PCW (New World)

By the time of 8512 CP/M version 1.7, the XBIOS memory map had been rearranged to support loadable drivers. This organisation was subsequently used by BIOS 2.1 on the PCW 9512 and all subsequent releases.

I have seen two versions of 1.7: 1.7, which can't load drivers, and 1.7H, which can. The two are identical in all other aspects. My guess is that at this time, the BIOS could be built in two versions: vanilla, which couldn't load drivers, and 'H', which could. Likewise, BIOS 2.1 can't load drivers; according to my theory, 2.1H, if it ever existed, would have been able to.

The addresses in this table come from BIOS 2.9. Other versions differ.

The new layout keeps the kernel in the bottom 16k (where it needs to be in order to access other memory banks) but pushes all the hardware drivers up so that they grow down from 58E0h — and, importantly, come down below 4000h. This is because, when disc transfers are done, the area from 4000h-7FFFh contains the source/target memory. Since any bank could be present in that area when the driver is called, the driver itself can't be allowed to occupy that space. Having the built-in CP/M drivers live there gets round the problem nicely.

(Note that NMI, which handles data transfers for the built-in floppy driver, is not in that area. It's down at the bottom of memory with the kernel).

Loadable device drivers cause a big change to the undocumented areas of the XBIOS jumpblock. Previously, the BIOS would do character I/O by calling the jumpblock with a device number. Now, it looks up the device's character device jumpblock in the table of character devices at 58E0h, and jumps straight to it.

At boot time, a loadable device manager is present in memory (for BIOS 2.9, at 0EDA0h; this varies depending on version). The device manager searches the boot disc for files ending in .FID. If any are found, it loads them into memory and allows them to add themselves to the BIOS as a character or disc device. The XBIOS registers its built-in character devices (screen, keyboard etc.) with this manager at boot time.

The Spectrum +3

Spectrum +3 CP/M appears to have branched off the PCW tree at about version 2.8 (with a few fragments, such as scrolling error messages, coming from CPC CP/M).

Like the 'new-world' PCW XBIOS, the Spectrum XBIOS comes in two parts, with the first half containing the jumpblock and messages, and the second half holding device drivers. Though the two parts are stored separately in the EMS file, they end up in a contiguous block of memory.

The major difference in +3 CP/M is that there is no KL (kernel) module. Rather than individual modules registering timers, event blocks, interrupt handlers and so forth, the 50Hz ticker interrupt handler is hardwired to perform the necessary functions.

Zero Page

In PCW CP/M, the first 128 bytes of bank 0 are laid out like this:

	DEFW	address
	DEFB	bank to page in at 4000h
	DEFB	bank to page in at 8000h

For comparison, the same area of memory in Spectrum +3 CP/M looks like this:

Disk Parameter Headers

Amstrad CP/M uses an extended version of the standard CP/M Plus DPH. There are two variants, for Old World and New World memory layouts:

Old World

In an Old World memory layout, the standard DPH is preceded by ten bytes. These match the Extended Disk Parameter Header in the Digital Research sample BIOS, described in §4.7.3 of the CP/M 3 System Guide:

-00A		Word	Write function
-008		Word	Read function
-006		Word	Select function
-004		Word	Init function
-002		Byte	Physical drive number (floppies only)
-001		Byte	Drive type (always 0?)
0000-0018		Standard CP/M Plus DPH

The first three functions should return results in the same format as their BIOS counterparts WRITE, READ and SELDSK respectively. On entry IX will hold the address of the drive DPB, and DE will hold the address of the DPH.

The fourth function appears to have been intended for drive initialisation, but is never called.

The track, sector and transfer address used by these functions are in a data structure at 0BEF0h [CPC] or 0BDF0h [PCW]. This corresponds to the structure beginning @adrv in the CP/M 3 sample BIOS:

0	DB	@adrv	;Selected logical drive
1	DB	@rdrv	;Selected physical drive (from the byte at DPH-2)
2	DW	@trk	;Track passed to SETTRK.
4	DW	@eect	;Sector passed to SETSEC.
6	DW	@dma	;DMA address passed to SETDMA.
8	DB	@cnt	;Multiple I/O count passed to MULTIO
9	DB	@dbnk	;DMA bank passed to SETBNK

New World

In the New World memory layout, the list of functions is replaced by a single reference to a jump table:

-004		Word	Address of Block device jump table
-002		Byte	Physical drive number (floppies only)
-001		Byte	Drive type (always 0FFh?)
0000-0018		Standard CP/M Plus DPH

A rough outline of development

Based on the CP/M versions I have, here are my best guesses at the order of the versions, and what changed when:

CPC 1.0: 1985

The original.

PCW 1.1: 1985

The first version I have for the PCW, though it's likely that version 1.0 existed as well.

PCW 1.2: 1985

Most of the changes appear to be to support the creation of foreign-language CP/M versions:

• The keystrokes used for Retry, Ignore and Cancel can now be localised; in 1.1 they were hardcoded.
• The default screen language and the default matrix printer language are localised, rather than hardcoded.
• System messages can contain accented characters.
• Some changes to the keyboard layout. Shift+STOP now generates expansion token 80h, not a literal Control-C; and Alt+STOP is ignored, rather than generating ESCape.
• Some tweaks to paper feeds in the dot-matrix printer driver.

PCW 1.4: 1985

The most common version for the 8256/8512, supplied with many of the machines, and was used to provide illustrations for the manual.

• Change to the dot-matrix printer font: The " and ' characters are chisel-like, not curly.
• The CPS8256 can now be driven in interrupt mode.
• The floppy driver uses a semaphore to signal completion, not a simple on/off flag.
• There is a kernel panic (RST 30h) which is invoked if the kernel runs out of memory or a process descriptor gets corrupted.

PCW 1.5: ~1985

This has only been seen in localised form for the USA - although it signs on as 1.5, the version number returned by CD VERSION is still 1.4. The only differences from stock 1.4 are data changes giving a modified keyboard layout and dot-matrix printer settings for the US, and changing the spelling of 'disc' to 'disk'.

PCW 1.7: 1986

• New World memory layout.
• New escape code in the terminal emulator: ESC @ (insert character)

PCW 1.7H: 1986

Supplied with the ASD HD20 hard drive.

• Includes code to load external device drivers (*.FID)
• In all other respects identical to 1.7.

PCW 2.1: 1987

Supplied with the PCW9512.

• Support added for the PCW9512 and its daisywheel printer.
• Does not include code to load *.FID drivers.
• Floppy drive support altered to support a system with a 720k drive A:. Also some tweaks made to floppy interrupt handling, perhaps to cope with slight hardware differences.

Spectrum 1.0: 1988

My best guess is that +3 CP/M was derived from the PCW CP/M codebase somewhere around here.

PCW 2.9: 1989

• Now supports the PCW9512 sheet feeder.
• FIDs now enabled again.
• The "Drive is A:" message moves four characters to the left (!)
• New ESC 4 1 / ESC 4 0 sequences to disable / enable screen scrolling.

PCW 1.11 / 2.11: 1991

Supplied with the PcW 9256 (1.11) and the PcW 9512+ (2.11).

• Somewhere between 1.7 and 1.11, the 1.x and 2.x version numbers were synchronised. This probably means some 2.x versions were skipped. My guess is that 1.7 corresponds to 2.0, 1.8 to 2.1, and the sync was done at 1.9 = 2.9.
• The dot-matrix printer driver allows redefinition of arbitrary characters, using jumps at FFF9h and FFFCh.
• Support added for second-generation PCWs: 9256, 9512+.
• Support added for booting the LocoScript installer (insert the boot floppy and press SHIFT+EXTRA+RELAY. Only applies to second-generation PCWs).
• Allows the floppy drives to be stepped at different rates.

PCW 1.12 / 2.12: 1992

Supplied with the PcW10.

• Support added for the standalone Centronics port accessory.

PCW 1.14 / 2.14: 1993

• Support for loading drive parameters from *.FIB files.
• Improved Centronics port detection.
• If the memory test reports 256k, reset the memory read/write register and try again.
• SHIFT+EXTRA+RELAY now works on first-generation PCWs as well.

PCW 1.15 / 2.15: 1993

• A single change: When doing a SHIFT+EXTRA+RELAY reboot, takes all devices out of interrupt mode so that they don't interrupt at an awkward moment.

Some versions I haven't got, but which probably existed:

• PCW 1.0: Screenshots in the PCW8256 manual at archive.org show J10CPM3.EMS, which would correspond to a 1.0 release.
• M14CPM3: Mentioned in issue 0 of Locomotive's Script newsletter. Locomotive had not heard of it, and suspected it was an unofficial modification to support an SCA printer interface.
• PCW 1.6: I'd guess this was released to support early hard drives such as the Timatic Winchester (possibly with hardware-specific drivers rather than the generic FID mechanism).
• PCW 1.8: Mentioned in '8000 Plus' as supplied with an add-on hard drive.
• PCW 2.0H: Seen in a screenshot in LocoScript's "Script" newsletter, issue 2 (January 1988). Interestingly, the sign-on message reports "54K TPA" rather than the usual "61K TPA", and the Mallard BASIC sign-on message also shows 7k less memory than would normally be the case. The 'H' suggests hard drive support, and the reduced TPA implies that 7k of common memory was being used for other purposes — possibly for the hard disk driver (rather than it being loaded as a .FID file in banked memory) or for development / debug tools. Also, the 2.x version number is associated with PCW 9512 CP/M, but the Mallard BASIC version is 1.29, distributed with 8000-series PCWs.
• PCW 2.5: Seen in a screenshot in '8000 Plus'; though the screenshot was inaccurate in at least one other respect, and so may not have been genuine.
• PCW 1.13 / 2.13: Mentioned online here.
• Spectrum 0.1: The screenshots in the Spectrum +3 manual show S01CPM3.EMS, which would likely have been an early build for internal use.

The modules in other systems

At least some of the modules are shared between CP/M and other Amstrad / Locomotive operating systems. LocoScript 1.20, for example, has its own versions of NMI, KL and DD, and so does the LocoLink server for the PCW. The main difference is that another module or set of modules (probably called "DOS") handles file access. The code in this module resembles +3DOS on the Spectrum +3 — or, rather, +3DOS was created from DOS, DD, and a few bits of MSG (and modified not to use the IY register, since on the Spectrum that's used for other purposes).

In LocoScript 1, at least, DOS appears to be split into two modules: one handling file operations (open, close, read, write etc) and the other abstracting the floppy drives and RAMdisc into generic block devices. Unlike CP/M it only supports three devices, hardcoded as A: B: and M:.

I suspect that SCR is also shared with LocoScript, since it has at least one feature (right-to-left text output) which is completely unused in CP/M.

The LocoLink server in LocoLink 2.02 is somewhere between versions 2.9 and 2.11 of the CP/M BIOS; it supports separate drive parameters for each drive, but not the Shift+Extra+Relay boot sequence.

John Elliott, 9 May 2018