IOMD ROM space saving

A small contribution.

ResourcesFS is unlike every other FS:

• it is more like a refined ROMFS from the BBC, and I do not think it owes much to FileSwitch.
• file paths are limited to a total of 63 characters and it has no directory structure, so it is not going to be a simple matter of replacing it with a compressed ImageFS.
• to my mind it is a very simple collection of data with a convenient filing front end.
• the pre-amble to the PRM chapter on ResourceFS is worth reading for the ideas prompting its use.

One thing to be wary of is that a fair number of OS/ROM components use OS_FSControl 21 to get a direct pointer to the uncompressed file content in ResourceFS.

For those that have not noticed, the !RunLink for Alarm etc uses it to set PAGE.

So care will be needed to avoid breaking those components (or any third-party software which does the same thing).

Indeed. ;-)

I would suggest Edit must be in ROM if you want to be able to mend a broken or missing Boot file that is stopping normal startup. Trying to load it from disc of any sort may be impossible. It has got me out of a hole several times! I have never neded Draw or Paint.

Yeah, “get out of trouble” apps are fair enough.

ResourcesFS is unlike every other FS

I forgot:

• a path name can contain characters &*%^@#.

I thought to try CObey in an IOMD ROM.

The builds are with DDE30a.

1. Vanilla IOMD

romlinker: Warning: Insufficient space for ROM symbols
romlinker: Image has 3005 bytes spare (2.93K)

2. IOMD with CObey replacing Obey

romlinker: ROM image overflowed by -272 bytes ... HeroNames

3. IOMD with CObey but Paint and Draw removed

romlinker: Image has 150567 bytes spare (147.04K)

This ROM is now running in RPCEmu.

2. IOMD with CObey replacing Obey
romlinker: ROM image overflowed by -272 bytes … HeroNames

What if you allow the image size to be bigger?
RPCEmu should be happy with that

Finally, an IOMD ROM with 32k HAL size, down from the current setting of 64K.

romlinker: Warning: Insufficient space for ROM symbols
romlinker: Image has 35709 bytes spare (34.87K)

How not to achieve this has kept me entertained for weeks now. I got the 32K HALSize file sorted but the builds never showed any increase in space. Prompted by an iomd-diet branch now in git I had another go today, and (missing) the penny dropped. The Kernel start address needs to be lowered, explicitly, to occupy the freed 32K.

HALSize needs specifying as 32K in the two Env files.

IOMD.IOMDHAL.RiscOS.Env.ROOL.IOMD32
IOMD.IOMDHAL.RiscOS.Env.ROOL.IOMD32/sh

An actual 32K HALSize file needs to be created and installed. (It is not present in DDE30a here.)

IOMD.IOMDHAL.RiscOS.Export.APCS-32.Hdr.Global.HALSize.32K

; HAL Size definition for 32K HAL

OSROM_HALSize           *       32*1024  ; HAL Size in bytes

                        END

To actually get the 32K space the Kernel start address needs to be lowered from &FC010000 to &FC008000 in the Component file.

# Kernel is first module in ROM seen as the UtilityModule
HAL_IOMD
Kernel                -at 0xFC008000

From the buld log.

Module name                Position in ROM  Module size
-----------                ---------------  -----------
HAL_IOMD                   FC000000         00002ED0
Kernel                     FC008000         00036D08
Podule                     FC03ED08         00001FA8

Either due to, or in spite of, all of the above the built 4MB ROM is running on RPCemu.