The Compaq Portable III

Hardware

Where the Portable and Portable II have a fairly similar design, with a desktop-ish ISA motherboard in a big case with a little CRT, the Portable III makes some more radical changes:

It's markedly smaller.
Instead of the CRT, there's a 640x400 LCD.
The motherboard is smaller and more specialised. There are no ISA slots, only two specialised connectors for daughterboards. One daughterboard handles video, the other supports an optional memory expansion and/or a modem.
In addition to the usual sockets on the back (serial, parallel and monitor) there's also a connector for an expansion unit, which contains the ISA slots.

Video output

The video hardware is a superset of CGA, with a doubled vertical resolution (640x400 rather than 640x200), outputting on an amber LCD.

There is no font ROM. The font (8x8 or 8x16) is held in another bank of video RAM, and loaded by the BIOS at boot time.

Some I/O ports used by the video hardware are:

13C6h [r/w]
	Bits 0-2: Master mode. 0=CGA  1=EGA  3=MDA  Other values unknown
	Bit 3:    Active monitor. Set for internal, reset for external.
		  When an external monitor is in use, the font is assumed 
		  to be 8x8; otherwise, it is 8x16.
	Bit 6:    If, when read, this is 0, then the Compaq extended video
                  functions (INT 10h / AX = 0BFh) are available.
17C6h [read]
	Video hardware capabilities. As described in the Interrupt List:

	Bit 0:    0 if CGA supported, else 1.
	Bits 1-2: Reserved (1)
	Bit 3:    0 if MDA supported, else 1.
	Bit 4:    0 if BitBlt engine supported, else 1.
	Bit 5:    0 if 132-column mode is available, else 1.
	Bit 6:    0 if 640x480x256 mode is available, else 1.
	Bit 7:    0 if 8-bit DAC is available, else 1.

	On my system, this returns 0E6h -- ie, CGA, MDA and BitBlt engine.

1BC6h [read]
	Bits 0-3: External monitor type
	Bits 4-7: Internal monitor type
		   0=> none
		   1=> dual-mode monitor
		   2=> CGA 640x200
		   3=> Compaq Color monitor
		   4=> 640x400 flat panel
                  (INT 10h/AX=BF03h describes other possible values, but 
                   these are VGA types not supported by the Portable).

23C6h [r/w]
	Bit 0:    Set for 400-line graphics mode. Reset for 200-line graphics
                  mode or for text.
	Bit 3:    Set to map the character generator RAM into memory where
                  the video RAM would normally be (at B8000 or B0000). The 
		  main font is at offset 0, the alternate font at offset 
		  1000h. Character cells are always 16 rows high; on an 
                  external CGA monitor, only the top 8 will be displayed.
	Bit 4:    Active intensity bit interpretation (toggled by INT 10h
                  with AX=1402h and BL=0B1h)
	Bits 5-7: LCD high intensity attribute is treated as:
		   0 => Ignore   
		   1 => Reverse video
		   2 => Underscore
		   3 => Alternate font
		   4 => Half intensity

27C6h [r/w]
	Bits 0-5: Plasma timeout in minutes, 0-3Fh.
	Bits 6-7: Unknown. Always 0 when written; masked off when read.

The Portable has two drive bays. One, which is the width of a normal 5.25" bay but not as high, contains a slimline high-density 5.25" floppy drive. The other, which is a full-sized bay, contains a 3.5" ATA hard drive on a shock-absorbing mount. The design of the case would also permit a twin-floppy configuration.

The BIOS supports only predefined drive geometries; if you want to use anything else, you'll need to use a disk manager overlay. For reference, the geometries are:

Type	Cylinders	Heads	Sectors	Capacity	Landing zone	Write precompensation	Control byte
1	306	4	17	10.2 M	305	128	0x00
2	615	4	17	20.4 M	638	128	0x00
3	615	6	17	30.6 M	615	128	0x00
4	1024	8	17	68.0 M	1023	512	0x00
5	940	6	17	46.8 M	939	512	0x00
6	697	5	17	28.9 M	696	128	0x00
7	462	8	17	30.7 M	511	256	0x00
8	925	5	17	38.4 M	924	128	0x00
9	900	15	17	112.1 M	899	-1	0x08
10	980	5	17	40.7 M	980	-1	0x00
11	925	7	17	53.7 M	924	128	0x00
12	925	9	17	69.1 M	924	128	0x08
13	612	8	17	40.6 M	611	256	0x00
14	980	4	17	32.5 M	980	128	0x00
15	0	0	0	0.0 M	0	0	0x00
16	612	4	17	20.3 M	612	0	0x00
17	980	5	17	40.7 M	980	128	0x00
18	966	6	17	48.1 M	966	128	0x00
19	1023	8	17	67.9 M	1023	-1	0x00
20	733	5	17	30.4 M	732	256	0x00
21	733	7	17	42.6 M	732	256	0x00
22	805	6	17	40.1 M	805	-1	0x00
23	924	8	17	61.4 M	924	-1	0x00
24	966	14	17	112.3 M	966	-1	0x08
25	966	16	17	128.3 M	966	-1	0x08
26	1023	14	17	118.9 M	1023	-1	0x08
27	966	10	17	80.2 M	966	-1	0x08
28	748	16	17	99.3 M	748	-1	0x08
29	805	6	26	61.3 M	805	-1	0x00
30	615	4	25	30.0 M	615	128	0x00
31	615	8	25	60.1 M	615	128	0x00
32	905	9	25	99.4 M	905	128	0x08
33	748	8	34	99.3 M	748	-1	0x00
34	966	7	34	112.3 M	966	-1	0x00
35	966	8	34	128.3 M	966	-1	0x00
36	966	9	34	144.3 M	966	-1	0x08
37	966	5	34	80.2 M	966	-1	0x00
38	611	16	63	300.7 M	611	-1	0x08
39	1023	11	33	181.3 M	1023	-1	0x08
40	1023	15	34	254.8 M	1023	-1	0x08
41	1023	15	33	247.3 M	1023	-1	0x08
42	1023	16	63	503.5 M	1023	-1	0x08
43	805	4	26	40.9 M	805	-1	0x00
44	805	2	26	20.4 M	805	-1	0x00
45	748	8	33	96.4 M	748	-1	0x00
46	748	6	33	72.3 M	748	-1	0x00
47	966	5	25	59.0 M	966	128	0x00

Keyboard

The keyboard controller (part number 106436-001) is probably the same one used in the Portable II. It supports all the functions of IBM's AT keyboard controller, plus a few of its own, mainly to set the processor speed.

XT keyboard compatibility

The original IBM AT keyboard controller has a backward-compatible mode where it supports an XT keyboard. This has to be selected manually. There are then three modes it can operate in:

AT keyboard, scancode translation from Set 3 to Set 1
AT keyboard, no scancode translation (native Set 3 codes)
XT keyboard, no scancode translation (native Set 1 codes)

The Compaq keyboard controller goes further, by auto-sensing the keyboard type. If an XT keyboard is connected but the IBM-compatible 'use XT protocol' option has not been selected, it will make the keyboard appear to be an AT keyboard, simulating responses to commands such as 'set LEDs' or 'set repeat rate'. Depending what keyboard is connected, it can operate in five modes:

AT keyboard, scancode translation from Set 3 to Set 1
AT keyboard, no scancode translation (native Set 3 codes)
XT keyboard (selected manually), no scancode translation (native Set 1 codes)
XT keyboard (simulating AT), no scancode translation (native Set 1 codes)
XT keyboard (simulating AT), reverse scancode translation (translates from Set 1 to Set 3)

The difference between modes 3 and 4 is how the keyboard is presented to the PC. In mode 3, the PC knows it has an XT keyboard. In mode 4, the PC thinks it has an AT keyboard (unless it queries the real keyboard type with command 0A5h below).

Keyboard controller commands

For more details see table P0401 in the Interrupt List, file PORTS.A:

0A1h

Set 6MHz speed

0A2h

Set 8MHz speed

0A3h

Set 16MHz speed

0A4h

Toggle CPU speed

0A5h

Special read (get keyboard type and other values). In the value returned:

	Bit    5 set for XT keyboard, clear for AT.
	Bits 2,3 give CPU speed.
		 Bits 2 and 3 set:  8 MHz
		 Bit  2 set:       16 MHz
		 Else:              6 MHz (if available; see below).

0A6h

Set CPU speed limit. Followed by one byte giving the new limit (0 or 1).

On some motherboards(?), port 86h also seems to be involved in CPU speed switching. When read:

	If bits 6 and 7 are set, CPU speed is 16MHz, slowing to 8MHz during 
                              floppy access.
	If bit 3 is set,   8MHz and 16MHz are available.
		    clear, 6MHz, 8MHz and 16MHz are available.

When the CPU speed is being set, this port is used if the value read had bit 3 set. Various permuations of bits 5,6,7 are then used to select the speed.

Firmware

The Portable III has 32k of BIOS in two 16k chips.

The Compaq ROM signature is a 6-byte string at F000:FFEAh: "COMPAQ". This is preceded by the product class signature at F000:FFE8h: "01".
The BIOS on mine is dated 29 January 1987; it's revision K.
As on the earlier Portables, the BIOS uses CTRL+ALT+< and CTRL+ALT+> to switch between internal and external monitors.
The video registers table (pointed to by INT 1Dh) is still only 64 bytes long, and therefore contains only the 6845 registers. The other bits of the table (video buffer sizes, columns on screen, etc.) are stored elsewhere in the ROM.
The Portable III no longer tries to launch BASICA.EXE when the ROM BASIC interrupt (INT 18h) is reached. Instead, it just displays the standard "Non-System Disk or Disk error" message, waits for a keypress, and reboots.
The CPU speed is set using INT 16h calls with AH=0F0h to 0F4h. See the Interrupt List for more details. The keystroke CTRL+ALT+backslash (on a UK keyboard, CTRL+ALT+hash) also toggles the turbo mode.
CTRL+ALT+minus, as on the Portable II, switches between MDA-compatible and CGA-compatible attributes in the 24x80 text modes (the hardware also supports this in the 24x40 text modes, but the BIOS doesn't).
The following extended Compaq video BIOS functions are suported on INT 10h:
0BF00h / 0BF01h
Select external / internal monitor

0BF02h
Set master mode of current controller

0BF03h
Get environment

0BF04h
Set mode switch delay (uses BL not BH)

0BF05h
Enable/disable display (uses BL not BH)

In addition, INT 10h / AH=14h (LCD functions) are supported, allowing the LCD font to be loaded and the behaviour of high intensity attributes to be set.

John Elliott 16 May 2009.