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BasiliskII/TECH
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gbeauche
Real addressing mode also works on Linux/ppc. Uhoh, what was the initial
03 окт 2002, 23:57
03 окт 2002, 23:578bf60db
BASILISK II TECHNICAL MANUAL
============================

0. Table of Contents
--------------------

  1. Introduction
  2. Modes of operation
  3. Memory access
  4. Calling native routines from 68k mode and vice-versa
  5. Interrupts
  6. Parts of Basilisk II
  7. Porting Basilisk II

1. Introduction
---------------

Basilisk II can emulate two kind of Macs, depending on the ROM being used:

  1. A Mac Classic
  2. A Mac II series computer ("Mac II series" here means all 68020/30/40
     based Macs with 32-bit clean ROMs (this excludes the original Mac II,
     the IIx/IIcx and the SE/030), except PowerBooks; in the following,
     "Mac II" is used as an abbreviation of "Mac II series computer", as
     defined above)

More precisely spoken, MacOS under Basilisk II behaves like on a Mac Classic
or Mac II because, apart from the CPU, the RAM and the ROM, absolutely no Mac
hardware is emulated. Rather, Basilisk II provides replacements (usually in
the form of MacOS drivers) for the parts of MacOS that access hardware. As
there are practically no Mac applications that access hardware directly (this
is also due to the fact that the hardware of different Mac models is sometimes
as different as, say, the hardware of an Atari ST and an Amiga 500), both the
compatibility and speed of this approach are very high.

2. Modes of operation
---------------------

Basilisk II is designed to run on many different hardware platforms and on
many different operating systems. To provide optimal performance under all
environments, it can run in four different modes, depending on the features
of the underlying environment (the modes are selected with the REAL_ADDRESSING,
DIRECT_ADDRESSING and EMULATED_68K defines in "sysdeps.h"):

1. Emulated CPU, "virtual" addressing (EMULATED_68K = 1, REAL_ADDRESSING = 0):
   This mode is designed for non-68k or little-endian systems or systems that
   don't allow accessing RAM at 0x0000..0x1fff. This is also the only mode
   that allows 24-bit addressing, and thus the only mode that allows Mac
   Classic emulation. The 68k processor is emulated with the UAE CPU engine
   and two memory areas are allocated for Mac RAM and ROM. The memory map
   seen by the emulated CPU and the host CPU are different. Mac RAM starts at
   address 0 for the emulated 68k, but it may start at a different address for
   the host CPU.

   In order to handle the particularities of each memory area (RAM, ROM and
   Frame Buffer), the address space of the emulated 68k is broken down into
   banks. Each bank is associated with a series of pointers to specific
   memory access functions that carry out the necessary operations (e.g.
   byte-swapping, catching illegal writes to memory). A generic memory access
   function, get_long() for example, goes through the table of memory banks
   (mem_banks) and fetches the appropriate specific memory access fonction,
   lget() in our example. This slows down the emulator, of course.

2. Emulated CPU, "direct" addressing (EMULATED_68K = 1, DIRECT_ADDRESSING = 1):
   As in the virtual addressing mode, the 68k processor is emulated with the
   UAE CPU engine and two memory areas are set up for RAM and ROM. Mac RAM
   starts at address 0 for the emulated 68k, but it may start at a different
   address for the host CPU. Besides, the virtual memory areas seen by the
   emulated 68k are separated by exactly the same amount of bytes as the
   corresponding memory areas allocated on the host CPU. This means that
   address translation simply implies the addition of a constant offset
   (MEMBaseDiff). Therefore, the memory banks are no longer used and the
   memory access functions are replaced by inline memory accesses.

3. Emulated CPU, "real" addressing (EMULATED_68K = 1, REAL_ADDRESSING = 1):
   This mode is intended for non-68k systems that do allow access to RAM
   at 0x0000..0x1fff. As in the virtual addressing mode, the 68k processor
   is emulated with the UAE CPU engine and two areas are allocated for RAM
   and ROM but the emulated CPU lives in the same address space as the host
   CPU. This means that if something is located at a certain address for
   the 68k, it is located at the exact same address for the host CPU. Mac
   addresses and host addresses are the same. The memory accesses of the CPU
   emulation still go through access functions but the address translation
   is no longer needed. The memory access functions are replaced by direct,
   inlined memory accesses, making for the fastest possible speed of the
   emulator. On little-endian systems, byte-swapping is still required, of
   course.

   A usual consequence of the real addressing mode is that the Mac RAM doesn't
   any longer begin at address 0 for the Mac and that the Mac ROM also is not
   located where it usually is on a real Mac. But as the Mac ROM is relocatable
   and the available RAM is defined for MacOS by the start of the system zone
   (which is relocated to the start of the allocated RAM area) and the MemTop
   variable (which is also set correctly) this is not a problem. There is,
   however, one RAM area that must lie in a certain address range. This area
   contains the Mac "Low Memory Globals" which (on a Mac II) are located at
   0x0000..0x1fff and which cannot be moved to a different address range.
   The Low Memory Globals constitute of many important MacOS and application
   global variables (e.g. the above mentioned "MemTop" variable which is
   located at 0x0108). For the real addressing mode to work, the host CPU
   needs access to 0x0000..0x1fff. Under most operating systems, this is a
   big problem. On some systems, patches (like PrepareEmul on the Amiga or
   the sheep_driver under BeOS) can be installed to "open up" this area. On
   other systems, it might be possible to use access exception handlers to
   emulate accesses to this area. But if the Low Memory Globals area cannot
   be made available, using the real addressing mode is not possible.
   
   Note: currently, real addressing mode is known to work only on AmigaOS,
   NetBSD/m68k, FreeBSD/i386, Linux/ppc and Linux/i386.

4. Native CPU (EMULATED_68K = 0, this also requires REAL_ADDRESSING = 1)
   This mode is designed for systems that use a 68k (68020 or better) processor
   as host CPU and is the technically most difficult mode to handle. The Mac
   CPU is no longer emulated (the UAE CPU emulation is not needed) but MacOS
   and Mac applications run natively on the existing 68k CPU. This means that
   the emulator has its maximum possible speed (very close to that of a real
   Mac with the same CPU). As there is no control over the memory accesses of
   the CPU, real addressing mode is implied, and so the Low Memory area must
   be accessible (an MMU might be used to set up different address spaces for
   the Mac and the host, but this is not implemented in Basilisk II). The
   native CPU mode has some possible pitfalls that might make its
   implementation difficult on some systems:
     a) Implied real addressing (this also means that Mac programs that go out
        of control can crash the emulator or the whole system)
     b) MacOS and Mac applications assume that they always run in supervisor
        mode (more precisely, they assume that they can safely use certain
        priviledged instructions, mostly for interrupt control). So either
        the whole emulator has to be run in supervisor mode (which usually is
        not possible on multitasking systems) or priviledged instructions have
        to be trapped and emulated. The Amiga and NetBSD/m68k versions of
        Basilisk II use the latter approach (it is possible to run supervisor
        mode tasks under the AmigaOS multitasking kernel (ShapeShifter does
        this) but it requires modifying the Exec task switcher and makes the
        emulator more unstable).
     c) On multitasking systems, interrupts can usually not be handled as on
        a real Mac (or with the UAE CPU). The interrupt levels of the host
        will not be the same as on a Mac, and the operating systems might not
        allow installing hardware interrupt handlers or the interrupt handlers
        might have different stack frames and run-time environments than 68k
        hardware interrupts. The usual solution is to use some sort of software
        interrupts or signals to interrupt the main emulation process and to
        manually call the Mac 68k interrupt handler with a faked stack frame.
     d) 68060 systems are a small problem because there is no Mac that ever
        used the 68060 and MacOS doesn't know about this processor. Basilisk II
        reports the 68060 as being a 68040 to the MacOS and patches some places
        where MacOS makes use of certain 68040-specific features such as the
        FPU state frame layout or the PTEST instruction. Also, Basilisk II
        requires that all of the Motorola support software for the 68060 to
        emulate missing FPU and integer instructions and addressing modes is
        provided by the host operating system (this also applies to the 68040).
     e) The "EMUL_OP" mechanism described below requires the interception and
        handling of certain emulator-defined instructions.

3. Memory access
----------------

There is often a need to access Mac RAM and ROM inside the emulator. As
Basilisk II may run in "real" or "virtual" addressing mode on many different
architectures, big-endian or little-endian, certain platform-independent
data types and functions are provided:

  a) "sysdeps.h" defines the types int8, uint8, int16, uint16, int32 and uint32
     for numeric quantities of a certain signedness and bit length
  b) "cpu_emulation.h" defines the ReadMacInt*() and WriteMacInt*() functions
     which should always be used to read from or write to Mac RAM or ROM
  c) "cpu_emulation.h" also defines the Mac2HostAddr() function that translates
     a Mac memory address to a (uint8 *) in host address space. This allows you
     to access larger chunks of Mac memory directly, without going through the
     read/write functions for every access. But doing so you have to perform
     any needed endianess conversion of the data yourself by using the ntohs()
     etc. macros which are available on most systems or defined in "sysdeps.h".

4. Calling native routines from 68k mode and vice-versa
-------------------------------------------------------

An emulator like Basilisk II requires two kinds of cross-platform function
calls:

  a) Calling a native routine from the Mac 68k context
  b) Calling a Mac 68k routine from the native context

Situation a) arises in nearly all Basilisk drivers and system patches while
case b) is needed for the invocation of Mac call-back or interrupt routines.
Basilisk II tries to solve both problems in a way that provides the same
interface whether it is running on a 68k or a non-68k system.

4.1. The EMUL_OP mechanism
--------------------------

Calling native routines from the Mac 68k context requires breaking out of the
68k emulator or interrupting the current instruction flow and is done via
unimplemented 68k opcodes (called "EMUL_OP" opcodes). Basilisk II uses opcodes
of the form 0x71xx (these are invalid MOVEQ opcodes) which are defined in
"emul_op.h". When such an opcode is encountered, whether by the emulated CPU
or a real 68k, the execution is interrupted, all CPU registers saved and the
EmulOp() function from "emul_op.cpp" is called. EmulOp() decides which opcode
caused the interrupt and performs the required actions (mostly by calling other
emulator routines). The EMUL_OP handler routines have access to nearly all of
the 68k user mode registers (exceptions being the PC, A7 and SR). So the
EMUL_OP opcodes can be thought of as extensions to the 68k instruction set.
Some of these opcodes are used to implement ROM or resource patches because
they only occupy 2 bytes and there is sometimes not more room for a patch.

4.2. Execute68k()
-----------------

"cpu_emulation.h" declares the functions Execute68k() and Execute68kTrap() to
call Mac 68k routines or MacOS system traps from inside an EMUL_OP handler
routine. They allow setting all 68k user mode registers (except PC and SR)
before the call and examining all register contents after the call has
returned. EMUL_OP and Execute68k() may be nested, i.e. a routine called with
Execute68k() may contain EMUL_OP opcodes and the EMUL_OP handlers may in turn
call Execute68k() again.

5. Interrupts
-------------

Various parts of Basilisk II (such as the Time Manager and the serial driver)
need an interrupt facility to trigger asynchronous events. The MacOS uses
different 68k interrupt levels for different events, but for simplicity
Basilisk II only uses level 1 and does it's own interrupt dispatching. The
"InterruptFlags" contains a bit mask of the pending interrupts. These are the
currently defined interrupt sources (see main.h):

  INTFLAG_60HZ    - MacOS 60Hz interrupt (unlike a real Mac, we also handle
                    VBL interrupts and the Time Manager here)
  INTFLAG_1HZ     - MacOS 1Hz interrupt (updates system time)
  INTFLAG_SERIAL  - Interrupt for serial driver I/O completion
  INTFLAG_ETHER   - Interrupt for Ethernet driver I/O completion and packet
                    reception
  INTFLAG_AUDIO   - Interrupt for audio "next block" requests
  INTFLAG_TIMER   - Reserved for a future implementation of a more precise
                    Time Manager (currently not used)
  INTFLAG_ADB     - Interrupt for mouse/keyboard input
  INTFLAG_NMI     - NMI for debugging (not supported on all platforms)

An interrupt is triggered by calling SetInterruptFlag() with the desired
interrupt flag constant and then TriggerInterrupt(). When the UAE 68k
emulator is used, this will signal a hardware interrupt to the emulated 680x0.
On a native 68k machine, some other method for interrupting the MacOS thread
has to be used (e.g. on AmigaOS, a signal exception is used). Care has to be
taken because with the UAE CPU, the interrupt will only occur when Basilisk II
is executing MacOS code while on a native 68k machine, the interrupt could
occur at any time (e.g. inside an EMUL_OP handler routine). In any case, the
MacOS thread will eventually end up in the level 1 interrupt handler which
contains an M68K_EMUL_OP_IRQ opcode. The opcode handler in emul_op.cpp will
then look at InterruptFlags and decide which routines to call.

6. Parts of Basilisk II
-----------------------

The conception of Basilisk II is quite modular and consists of many parts
which are relatively independent from each other:

 - UAE CPU engine ("uae_cpu/*", not needed on all systems)
 - ROM patches ("rom_patches.cpp", "slot_rom.cpp" and "emul_op.cpp")
 - resource patches ("rsrc_patches.cpp" and "emul_op.cpp")
 - PRAM Utilities replacement ("xpram.cpp")
 - ADB Manager replacement ("adb.cpp")
 - Time Manager replacement ("timer.cpp")
 - SCSI Manager replacement ("scsi.cpp")
 - video driver ("video.cpp")
 - audio component ("audio.cpp")
 - floppy driver ("sony.cpp")
 - disk driver ("disk.cpp")
 - CD-ROM driver ("cdrom.cpp")
 - external file system ("extfs.cpp")
 - serial drivers ("serial.cpp")
 - Ethernet driver ("ether.cpp")
 - system-dependant device access ("sys_*.cpp")
 - user interface strings ("user_strings.cpp")
 - preferences management ("prefs.cpp" and "prefs_editor_*.cpp")

Most modules consist of a platform-independant part (such as video.cpp) and a
platform-dependant part (such as video_beos.cpp). The "dummy" directory
contains generic "do-nothing" versions of some of the platform-dependant
parts to aid in testing and porting.

6.1. UAE CPU engine
-------------------

All files relating to the UAE 680x0 emulation are kept in the "uae_cpu"
directory. The "cpu_emulation.h" header file defines the link between the
UAE CPU and the rest of Basilisk II, and "basilisk_glue.cpp" implements the
link. It should be possible to replace the UAE CPU with a different 680x0
emulation by creating a new "xxx_cpu" directory with an appropriate
"cpu_emulation.h" header file (for the inlined memory access functions) and
writing glue code between the functions declared in "cpu_emulation.h" and
those provided by the 680x0 emulator.

6.2. ROM and resource patches
-----------------------------

As described above, instead of emulating custom Mac hardware, Basilisk II
provides replacements for certain parts of MacOS to redirect input, output
and system control functions of the Mac hardware to the underlying operating
systems. This is done by applying patches to the Mac ROM ("ROM patches") and
the MacOS system file ("resource patches", because nearly all system
software is contained in MacOS resources). Unless resources are written back
to disk, the system file patches are not permanent (it would cause many
problems if they were permanent, because some of the patches vary with
different versions of Basilisk II or even every time the emulator is
launched).

ROM patches are contained in "rom_patches.cpp" and resource patches are
contained in "rsrc_patches.cpp". The ROM patches are far more numerous
because nearly all the software needed to run MacOS is contained in the Mac
ROM (the system file itself consists mainly of ROM patches, in addition to
pictures and text). One part of the ROM patches involves the construction of
a NuBus slot declaration ROM (in "slot_rom.cpp") which is used to add the
video and Ethernet drivers. Apart from the CPU emulation, the ROM and
resource patches contain most of the "logic" of the emulator.

6.3. PRAM Utilities
-------------------

MacOS stores certain nonvolatile system parameters in a 256 byte battery
backed-up CMOS RAM area called "Parameter RAM", "PRAM" or "XPRAM" (which
refers to "Extended PRAM" because the earliest Mac models only had 20 bytes
of PRAM). Basilisk II patches the ClkNoMem() MacOS trap which is used to
access the XPRAM (apart from some routines which are only used early during
system startup) and the real-time clock. The XPRAM is emulated in a 256 byte
array which is saved to disk to preserve the contents for the next time
Basilisk is launched.

6.4. ADB Manager
----------------

For emulating a mouse and a keyboard, Basilisk II patches the ADBOp() MacOS
trap. Platform-dependant code reports mouse and keyboard events with the
ADBMouseDown() etc. functions where they are queued, and the INTFLAG_ADB
interrupt is triggered. The ADBInterrupt() handler function sends the input
events to MacOS by calling the ADB mouse and keyboard handlers with
Execute68k().

6.5. Time Manager
-----------------

Basilisk II completely replaces the Time Manager (InsTime(), RmvTime(),
PrimeTime() and Microseconds() traps). A "TMDesc" structure is associated
with each Time Manager task, that contains additional data. The tasks are
executed in the TimerInterrupt() function which is currently called inside
the 60Hz interrupt handler, thus limiting the resolution of the Time Manager
to 16.6ms.

6.6. SCSI Manager
-----------------

The (old-style) SCSI Manager is also completely replaced and the MacOS
SCSIDispatch() trap redirected to the routines in "scsi.cpp". Under the
MacOS, programs have to issue multiple calls for all the different phases of
a SCSI bus interaction (arbitration, selection, command transfer etc.).
Basilisk II maps this API to an atomic API which is used by most modern
operating systems. All action is deferred until the call to SCSIComplete().
The TIB (Transfer Instruction Block) mini-programs used by the MacOS are
translated into a scatter/gather list of data blocks. Operating systems that
don't support scatter/gather SCSI I/O will have to use buffering if more
than one data block is being transmitted. Some more advanced (but rarely
used) aspects of the SCSI Manager (like messaging and compare operations)
are not emulated.

6.7. Video driver
-----------------

The NuBus slot declaration ROM constructed in "slot_rom.cpp" contains a
driver definition for a video driver. The Control and Status calls of this
driver are implemented in "video.cpp".

The host-side initialization of the video system is done in VideoInit().
This function must fill the VideoModes vector with a list of supported video
modes (combinations of color depth and resolution). It must then call
video_init_depth_list() and setup the VideoMonitor structure with the
default mode information and the address of a frame buffer for MacOS. In
real addressing mode, this frame buffer must be in a MacOS compatible layout
(big-endian and 1, 2, 4 or 8 bits paletted chunky pixels, RGB 5:5:5 or xRGB
8:8:8:8). In virtual addressing mode, the frame buffer is located at address
0xa0000000 on the Mac side and you have to supply the host address, size and
layout (BasiliskII will do an automatic pixel format conversion in virtual
addressing mode) in the variables MacFrameBaseHost, MacFrameSize and
MacFrameLayout.

There are two functions of the platform-dependant video driver code that get
called during runtime: video_set_palette() to update the CLUT (for indexed
modes) or gamma table (for direct color modes), and video_switch_to_mode()
to switch to a different color depth and/or resolution (in this case the
frame buffer base in VideoMonitor must be updated).

6.8. Audio component
--------------------

Basilisk II provides a Sound Manager 3.x audio component for sound output.
Earlier Sound Manager versions that don't use components but 'snth'
resources are not supported. Nearly all component functions are implemented
in "audio.cpp". The system-dependant modules ("audio_*.cpp") handle the
initialization of the audio hardware/driver, volume controls, and the actual
sound output.

The mechanism of sound output varies depending on the platform but usually
there will be one "streaming thread" (either a thread that continuously
writes data buffers to the audio device or a callback function that provides
the next data buffer) that reads blocks of sound data from the MacOS Sound
Manager and writes them to the audio device. To request the next data
buffer, the streaming thread triggers the INTFLAG_AUDIO interrupt which will
cause the MacOS thread to eventually call AudioInterrupt(). Inside
AudioInterrupt(), the next data block will be read and the streaming thread
is signalled that new audio data is available.

6.9. Floppy, disk and CD-ROM drivers
------------------------------------

Basilisk II contains three MacOS drivers that implement floppy, disk and
CD-ROM access ("sony.cpp", "disk.cpp" and "cdrom.cpp"). They rely heavily on
the functionality provided by the "sys_*.cpp" module. BTW, the name ".Sony"
of the MacOS floppy driver comes from the fact that the 3.5" floppy drive in
the first Mac models was custom-built for Apple by Sony (this was one of the
first applications of the 3.5" floppy format which was also invented by
Sony).

6.10. External file system
--------------------------

Basilisk II also provides a method for accessing files and direcories on the
host OS from the MacOS side by means of an "external" file system
(henceforth called "ExtFS"). The ExtFS is built upon the File System Manager
1.2 interface that is built into MacOS 7.6 (and later) and available as a
system extension for earlier MacOS versions. Unlike other parts of Basilisk
II, extfs.cpp requires POSIX file I/O and this is not going to change any
time soon, so if you are porting Basilisk II to a system without POSIX file
functions, you should emulate them.

6.11. Serial drivers
--------------------

Similar to the disk drivers, Basilisk II contains replacement serial drivers
for the emulation of Mac modem and printer ports. To avoid duplicating code,
both ports are handled by the same set of routines. The SerialPrime() etc.
functions are mostly wrappers that determine which port is being accessed.
All the real work is done by the "SERDPort" class which is subclassed by the
platform-dependant code. There are two instances (for port A and B) of the
subclasses.

Unlike the disk drivers, the serial driver must be able to handle
asynchronous operations. Calls to SerialPrime() will usually not actually
transmit or receive data but delegate the action to an independant thread.
SerialPrime() then returns "1" to indicate that the I/O operation is not yet
completed. The completion of the I/O request is signalled by calling the
MacOS trap "IODone". However, this can't be done by the I/O thread because
it's not in the right run-time environment to call MacOS functions.
Therefore it will trigger the INTFLAG_SERIAL interrupt which causes the
MacOS thread to eventually call SerialInterrupt(). SerialInterrupt(), in
turn, will not call IODone either but install a Deferred Task to do the job.
The Deferred Task will be called by MacOS when it returns to interrupt level
0. This mechanism sounds complicated but is necessary to ensure stable
operation of the serial driver.

6.12. Ethernet driver
---------------------

A driver for Ethernet networking is also contained in the NuBus slot ROM.
Only one ethernet card can be handled by Basilisk II. For Ethernet to work,
Basilisk II must be able to send and receive raw Ethernet packets, including
the 14-byte header (destination and source address and type/length field),
but not including the 4-byte CRC. This may not be possible on all platforms
or it may require writing special net drivers or add-ons or running with
superuser priviledges to get access to the raw packets.

For situations in which access to raw Ethernet packets is not possible,
Basilisk II implements a special "tunneling" mode in which it sends and
receives packets via UDP, using BSD socket functions. It simply wraps the
Ethernet packets into UDP packets, using dummy Ethernet addresses that are
made up of the IP address of the host. Ethernet broadcast and AppleTalk
multicast packets are sent to the IP broadcast address. Because of this
non-standard way of tunneling, it is only possible to set up a "virtual"
network amongst machines running Basilisk II in this way.

Writing packets works as in the serial drivers. The ether_write() routine
may choose to send the packet immediately (e.g. under BeOS) and return noErr
or to delegate the sending to a separate thread (e.g. under AmigaOS) and
return "1" to indicate that the operation is still in progress. For the
latter case, a Deferred Task structure is provided in the ether_data area to
call IODone from EtherInterrupt() when the packet write is complete (see
above for a description of the mechanism).

Packet reception is a different story. First of all, there are two methods
provided by the MacOS Ethernet driver API to read packets, one of which
(ERead/ ERdCancel) is not supported by Basilisk II. Basilisk II only
supports reading packets by attaching protocol handlers. This shouldn't be a
problem because the only network code I've seen so far that uses ERead is
some Apple sample code. AppleTalk, MacTCP, MacIPX, OpenTransport etc. all
use protocol handlers. By attaching a protocol handler, the user of the
Ethernet driver supplies a handler routine that should be called by the
driver upon reception of Ethernet packets of a certain type. 802.2 packets
(type/length field of 0..1500 in the packet header) are a bit special: there
can be only one protocol handler attached for 802.2 packets (by specifying a
packet type of "0"). The MacOS LAP Manager will attach a 802.2 handler upon
startup and handle the distribution of 802.2 packets to sub-protocol
handlers, but the Basilisk II Ethernet driver is not concerned with this.

When the driver receives a packet, it has to look up the protocol handler
installed for the respective packet type (if any has been installed at all)
and call the packet handler routine. This must be done with Execute68k()
from the MacOS thread, so an interrupt (INTFLAG_ETHER) is triggered upon
reception of a packet so the EtherInterrupt() routine can call the protocol
handler. Before calling the handler, the Ethernet packet header has to be
copied to MacOS RAM (the "ed_RHA" field of the ether_data structure is
provided for this). The protocol handler will read the packet data by means
of the ReadPacket/ReadRest routines supplied by the Ethernet driver. Both
routines will eventually end up in EtherReadPacket() which copies the data
to Mac address space. EtherReadPacket() requires the host address and length
of the packet to be loaded to a0 and d1 before calling the protocol handler.

Does this sound complicated? You are probably right. Here is another
description of what happens upon reception of a packet:
  1. Ethernet card receives packet and notifies some platform-dependant entity
     inside Basilisk II
  2. This entity will store the packet in some safe place and trigger the
     INTFLAG_ETHER interrupt
  3. The MacOS thread will execute the EtherInterrupt() routine and look for
     received packets
  4. If a packet was received of a type to which a protocol handler had been
     attached, the packet header is copied to ed_RHA, a0/d1 are loaded with
     the host address and length of the packet data, a3 is loaded with the
     Mac address of the first byte behing ed_RHA and a4 is loaded with the
     Mac address of the ed_ReadPacket code inside ether_data, and the protocol
     handler is called with Execute68k()
  5. The protocol handler will eventually try to read the packet data with
     a "jsr (a4)" or "jsr 2(a4)"
  6. This will execute an M68K_EMUL_OP_ETHER_READ_PACKET opcode
  7. The EtherReadPacket() opcode handling routine will copy the requested
     part of the packet data to Mac RAM using the pointer and length which are
     still in a0/d1

For a more detailed description of the Ethernet driver, see the book "Inside
AppleTalk".

6.13. System-dependant device access
------------------------------------

The method for accessing floppy drives, hard disks, CD-ROM drives and files
vary greatly between different operating systems. To make Basilisk II easily
portable, all device I/O is made via the functions declared in "sys.h" and
implemented by the (system-dependant) "sys_*.cpp" modules which provides a
standard, Unix-like interface to all kinds of devices.

6.14. User interface strings
----------------------------

To aid in localization, all user interface strings of Basilisk II are
collected in "user_strings.cpp" (for common strings) and
"user_strings_*.cpp" (for platform-specific strings), and accessed via the
GetString() function. This way, Basilisk II may be easily translated to
different languages.

6.15. Preferences management
----------------------------

The module "prefs.cpp" handles user preferences in a system-independant way.
Preferences items are accessed with the PrefsAdd*(), PrefsReplace*() and
PrefsFind*() functions and stored in human-readable and editable text files
on disk. There are two lists of available preferences items. The first one,
common_prefs_items, is defined in "prefs_items.cpp" and lists items which
are available on all systems. The second one, platform_prefs_items, is
defined in "prefs_*.cpp" and lists the prefs items which are specific to a
certain platform.

The "prefs_editor_*.cpp" module provides a graphical user interface for
setting the preferences so users won't have to edit the preferences file
manually.

7. Porting Basilisk II
----------------------

Porting Basilisk II to a new platform should not be hard. These are the
steps involved in the process:

1. Create a new directory inside the "src" directory for your platform. If
   your platform comes in several "flavours" that require adapted files, you
   should consider creating subdirectories inside the platform directory.
   All files needed for your port must be placed inside the new directory.
   Don't scatter platform-dependant files across the "src" hierarchy.
2. Decide in which mode (virtual addressing, real addressing or native CPU)
   Basilisk II will run.
3. Create a "sysdeps.h" file which defines the mode and system-dependant
   data types and memory access functions. Things which are used in Basilisk
   but missing on your platform (such as endianess macros) should also be
   defined here.
4. Implement the system-specific parts of Basilisk:
     main_*.cpp, sys_*.cpp, prefs_*.cpp, prefs_editor_*.cpp, xpram_*.cpp,
     timer_*.cpp, audio_*.cpp, video_*.cpp, serial_*.cpp, ether_*.cpp,
     scsi_*.cpp and clip_*.cpp
   You may want to take the skeleton implementations in the "dummy" directory
   as a starting point and look at the implementation for other platforms
   before writing your own.
5. Important things to remember:
     - Use the ReadMacInt*() and WriteMacInt*() functions from "cpu_emulation.h"
       to access Mac memory
     - Use the ntohs() etc. macros to convert endianess when accessing Mac
       memory directly
     - Don't modify any source files outside of your platform directory unless
       you really, really have to. Instead of adding "#ifdef PLATFORM" blocks
       to one of the platform-independant source files, you should contact me
       so that we may find a more elegant and more portable solution.
6. Coding style: indent -kr -ts4


Christian Bauer
<Christian.Bauer@uni-mainz.de>