>This is what I was looking for  ;-)  and having read isareg.h, it appears
>that the addresses I'm attempting to read are beyond IO_ISAEND (0x3ff),
>which would explain the result.

Just as an aside, have you enabled the extended registers in the Aladdin 
configuration space?  This is the code we use in Linux to do this:

#define CONFIG_PORT	0x370
#define INDEX_PORT	(CONFIG_PORT)
#define DATA_PORT	(CONFIG_PORT + 1)

	/* Set Aladdin to CONFIGURE mode */
	outb(0x51, CONFIG_PORT);
	outb(0x23, CONFIG_PORT);

	/* Select logical device 3 */
	outb(0x07, INDEX_PORT);
	outb(0x03, DATA_PORT);

	/* Set parallel port to DMA channel 3, ECP+EPP1.9, 
	   enable EPP timeout */
	outb(0x74, INDEX_PORT);
	outb(0x03, DATA_PORT);
	
	outb(0xf0, INDEX_PORT);
	outb(0x0f, DATA_PORT);

	outb(0xf1, INDEX_PORT);
	outb(0x07, DATA_PORT);

	/* Set Aladdin to RUN mode */
	outb(0xbb, CONFIG_PORT);

p.