HAL Device overview

Hardware Abstraction Layer

» HAL Device API

» Overview

The HAL introduces the new concept of a “device”. A device is a logical
representation of a component of hardware. Each active device is uniquely
identified by a constant pointer to a device descriptor. The descriptor is
a structure which contains information about the device and a series of entry
points to perform usually simple operations on the device. Devices can be
provided by the Bootloader, or by RISC OS modules.

Devices provided outside the Bootloader are, in principle, hot swappable,
although it is up to device drivers using it whether they can support this.

Throughout this document, device descriptors are described in terms of C,
although the scheme maps naturally to assembler or C++. All device calls use
the base ATPCS calling standard (R0-R3 arguments/return values, R4-R11
preserved, R12 corrupted), to permit straightforward use from C or assembler.

From C:

XXXDevice->Activate(XXXDevice);

A simple call to a activate a device from assembler might look like:

LDR    R0, XXXDevice
MOV    LR, PC
LDR    PC, [R0, #deventry_activate]      ; R0-R3,R12 corrupted

If an assembler device driver module is using a lot of device calls, it
might be preferable to move the workspace pointer from the traditional R12
to R11.

Information source: Kernel.Docs.HAL.NewAPI in CVS