English to English - PDF2TEXT - EXDOS System Specifications (early)

1. INTRODUCTION

The main code of EXDOS is the FILING SYSTEM HANDLER (FISH) which provides all the facilities necessary for communcacíng with fiLes on disk. User programs (apart fromm IS_DOS) do not communicate directly with this, but go through the EXDOS CLI or the DISK DEVICE which in turn calls FISH.

Since FISH is the central part of EXDOS through which all communication goes, this will be described in detail before covering the higher level functions such as the EXDOS CLI and the DISK DEVICE.

1.1 EXDOS ROM ENTRY POINTS

The EXDOS ROM is formatted as an EXOS extension ROM with two additional entry points to FISH and to DISKIO. The DISKIO entry point is provided for user's who wish to use the disk controller hardware directly without going through EXDOS. The FISH entry point is provided to enable IS-DOS to call FISH, and is not intended for user programs. There is of course the normal EXOS system extension entry point which provides various facilities including initialisation and the EXDOS CLI. The format of the start of the EXDOS ROM is as follows:

00h DB "EXOS_ROM"

08h DW 0

OAh JP SYS EXT

ODh JP DISKIO

1Gh JP FISH

2. INITIALISATION

During EXOS's cold start routine it will find the EXOOS ROM and include it in its list of extensíon ROMs. The main stages in EXDOS initialisation which then follow are as below:

1. EXOS calls the EXDOS ROM with action code 7 to ask it how much RAM it requires. EXDOS will ask for sufficient RAM in the system segment for all its internal variables and data areas, including those needed by UNITH, DISKIO and RAMUNIT. This includes three default sector buffers (512 bytes each) so the amount of RAM required here will be about 2.5...3k.

2. Next EXOS calls the EXDOS ROM with action code 8 to initialise it. EXDOS uses this to set up its internal data structures and to initialise its variables. UNITH ...