All disk I/O calls from ProDOS or GS/OS, regardless of them targeting a native ProDOS block device (i.e. the Apple Disk 3.5") or a Smartport device, use the Slot 5 MLI. This MLI provides two entry points for operating systems or applications that bypass the ProDOS application layer; the ProDOS and Smartport device drivers.
Under the hood, these calls are forwarded to the appropriate firmware handling either ProDOS block devices or the Smartport device. On Port 5 for example, calls are made to an entry function located somewhere in the internal firmware located in the $C500 block (see the Apple IIgs Firmware Reference Chapter 7 for specifics.)
From here on, we'll assume calls are made using entry points at the above mentioned $C500 firmware space. Also this document focuses on devices attached to an Apple IIgs, but it's likely most of this is applicable to older Apple IIs with relevant firmware and or cards.
Later Apple II hardware support a slot-agnostic interface to devices physically connected to the external disk I/O port. Apple //c and IIgs systems supply this physical port to daisy-chain both 5.25" and 3.5" drives. For the //c this is restrict to the Unidisk 3.5" due to a lack of 'fast' access.
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+-----+ | P | +-------------+ +--------------------+
| IWM |.......| O |=====| Apple |==================| 5.25" Disk II |
| | | R | | 3.5" Drives | | Drives |
+-----+ | T | +-------------+ +--------------------+
-----
Application programmers used to thinking of "Slots" 5 and 6 as physical card slots instead should consider these virtual slots accessed via the IWM (Integrated Woz Machine). The IWM itself is evenetually connected to the Mega II/MMC which allows applications to control disk I/O access independent of physical port.
Programmers used to accessing Disk I/O via the original BIT $C080,X type
instructions where X=#$60 can continue to do so (where signals are passed
to the IWM vs the Disk I/O card.). This is how old Apple II+/IIe software can
run on a IIc and IIgs without physical card slots for Disk I/O.
Like IDE, SCSI and later USB, Smartport allows multiple devices to coexist on the same physical bus.
Smartport is a legacy Apple standard for sending I/O between several devices on a single physical disk port. As far as I know was used solely on Apple II devices specifically to allow Apple IIs to use multiple drive types beyond the Disk II (and Apple 3.5 Disk.) At the same time, with Smartport it is possible for all of these drive types to coexist on the same physical connection, despite Disk II devices themselves not being Smartport compliant.
Unlike Disk II which offers direct access to the drive's motor and arm, Smartport aware devices typically implement a series of commands on the device that handle this for the host. These commands are issued by the OS to read, write data and query state.
NOTE: Certain devices like the Apple 3.5" operate outside of the Smartport standard. They have some "smart" features yet operate with less overhead and simpler onboard logic that Smartport devices like the Unidisk 3.5.
Smartport relies on compliant devices that cooperate with others on the physical disk port. Devices are daisy-chained as described below. This chaining allows devices farther up the chain to block I/O to devices down the chain.
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+-----+ | P | +-------------+ +-----------+ +---------------+
| IWM |.......| O |=====| Apple |====| Smartport |======| 5.25" Disk II |
| | | R | | 3.5" Drives | | Drive | | Drives |
+-----+ | T | +-------------+ +-----------+ +---------------+
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For example, when the Smartport Drive is functioning, I/O to the Disk II is blocked until the Smartport Drive releases its lock on the I/O lines. As a consequence, this means that any device before the Smartport can intercept communication on the bus. This limitation requires that the host strictly controls what device is active and when it's accessed. This control logic exists on the Smartport firmware or software device driver. ProDOS by design abstracts the enumeration of such devices, and its arbitration between user and firmware.
The firmware sends signals on the bus to inform Smartport devices that it's ready to read or write data. These signals are specifically selected to not conflict with Disk II operation. When the Smartport bus comes online, the signals that communicate this state have no practical effect on the Disk II drive. The Disk II will not receive signals as long as the bus is held by the Smartport device and host.
Note that the IWM uses a special data line to activate the Apple 3.5 Disk. This line was added in later hardware (Apple IIgs and the //c+) to support the Apple 3.5 Drive. Smartport devices generally did not use the 3.5 line and in the case of the Disk II was used as a ground line. Likely other Smartport devices do the same. This is why the Apple 3.5 drives are listed first in the daisy chain diagram above as to prevent other devices blocking the 3.5 line signal.
This design allows Disk II drives to coexist with Smartport devices. The details of why are discussed in the next section.
The SmartPort protocol is implemented between IWM and emulated storage. The Apple IIGS firmware reference provides thorough documentation on the protocol and packet types passed along the bus when enabled.
While an accurate emulation of SmartPort on disk port, there are limitations. Notably bus speed is limited to machine speed which runs at 1Mhz (it's unclear if fast IWM mode is supported but the ROM deals with slow mode here.)
Most emulators implement a Slot 7 "SmartPort". The card ROM at $C700 follows the SmartPort calling conventions so that ProDOS can interact with the device. But the actual calls are then forwarded to the emulator which copies block data from the emulated storage device back to the application. This approach has two major benefits:
- Speed: blocks are copied to and from the emulated machine's RAM directly
- Usability: Slot 5 prevents SmartPort devices from booting while a 3.5" drive is connected. Instead the boot ROM in Slot 7 will provide the bootloader for the emulated device.
Apple IIgs Firmware Reference
Apple IIgs Hardware Reference
http://mirrors.apple2.org.za/apple.cabi.net/FAQs.and.INFO/DiskDrives/drive.order.txt