MiniVNC is a remote desktop server that has been written from the ground up for best performance on 68k Macintosh computers. It was originally an experiment to see whether a Macintosh Plus could be controlled remotely, but has since been expanded to support color on all vintage color Macs!
MiniVNC is built on MacTCP and requires System 7, but it will operate on later Macs using Open Transport. MiniVNC has been developed and tested using a RaSCSI device operating as an Ethernet bridge, but also works using a Mac with a built-in Ethernet port.
The goal of MiniVNC is to provide better performance and compatibility with older Macs than ChromiVNC
It accomplishes these goals by:
- Using Classic Networking (i.e. MacTCP) rather than Open Transport
- Implementing a limited subset of the Remote Framebuffer Protocol, which favor of performance over full compatibility with all clients.
- Implements fast but inexact screen change detection, favoring performance and low memory use while allowing for the occasional missed updates and visual artifacts.
- Written in C++, with optimized 68x assembly code where necessary to achieve best performance.
Development of MiniVNC required months of experimentation and hacking. The following write-up provides details for developers who have an interest.
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On the Macintosh, the mouse can be progmattically changed by writing
the new position to the low-memory globals MouseTemp and
RawMouseLocation and then copying the value of CrsrCouple to
CursorNew to signal the change. This technique is borrowed from
ChromiVNC.
Mouse button control presented a challenge. The technique used by
ChromiVNC is to write the new mouse button state to the low-memory
global MBState while simultaneously posting a mouseUp or mouseDown
event to the System event queue. This works on modern Macs, but on the
Macintosh Plus it only works for clicking. Mouse dragging — and
crucially, menu selection — does not work. On that machine the
VIA interrupt in ROM constantly overwrites MBState with the button
state from the physical mouse, so the trick of writing a value to
MBState does not work.
I attempted patching the Button trap and implemented a journaling driver,
but the former was ineffective while the latter was found to be broken and
unusable under System 7's multi-tasking model.
At last, an analysis of the disassembled code for the VIA interrupt
revealed it deglitched the mouse button by waiting three ticks prior to
updating MBState. The low-memory variable MBTicks indicates the start
of the wait period and by periodically setting it to the future &emdash;
in advance of TickCount — I found I could keep the VIA routine
waiting indefinitely so that I could alter MBState at will. This
allowed me control of the mouse button on all Macs, including the Macintosh
Plus.
On the vintage Macintosh, the address of the framebuffer is stored in the
low-memory global ScrnBase. A crucial part of a VNC server is detecting
changes to the screen. While this could be done by maintaining a separate
copy of the framebuffer and comparing every pixel, this requires a lot of
memory and a lot of memory accesses.
In MiniVNC, I decided to compute a 32-bit sum across each row of pixels and a 32-bit sum up and down the screen. For both the horizontal and vertical sums, the new and old sums are compared to detect screen changes. It turns out this is also an quick way to determine the bounds of the change rectangle as the location of the first and last changed sum along an axis can be taken as the rectangle bounds along the axis.
This is an inexact approach which makes the server blind to many types of screen changes. Accuracy could be improved somewhat by using a positional checksum like a Fletcher's checksum, at the expense of more computation and storage.
An ordinary VNC server would attempt to minimize transmission by sending the smallest possible change rectangle. MiniVNC keeps the horizonal bounds of change rectangles aligned to byte boundaries. On the Macintosh Plus, which uses one bit per pixel, this is crucial as it allows screen data to be sent without any bit-shift or bit masking operations, which are particularly slow on the 68000.
When using a 32-bit column sum (as opposed to the XOR operation used in my first implementation), a change in one pixel column can cause a carry to the next, meaning that the horizontal bounds of change rectangles are further aligned to 32-bit boundaries.
On a B&W display, this causes change rectangle bounds to fall on 32 pixel increments, which might cause quite a lot of extra data to be transmitted.
The effect is minimized on color displays, which pack fewer pixels per byte. For example, on a 256 color display, the change rectangles can fall on 4 pixel boundaries, mimimizing the size of change rectangles.
The VNC protocol is meant for color computers and most encodings assume a color depth of at least eight bits. However, the TRLE encoding is unique in that it supports a paletted tile type that allows for a 1-bit, 2-bit and 4-bit encodings.
The ability to transmit 1-bit data is what caused me to require MiniVNC to use TRLE encoding exclusively. This does not meet the VNC specifications, but is compatible with modern clients. To support raw encoding, as required by the specifications, would necessitate expanding each 1-bit pixel in a byte into a full color byte, an operation which would be prohibitive on older Macs.
On the Macintosh Plus, I transmit all tiles using the 1-bit paletted tile type. This, together with byte alignment, allows the encoding process to be a straight copy full bytes without any bit shifting or masking.
As an optimization, during the copy I will detect if a tile consists of all zeros (i.e. white). If this is the case, I emit a solid white tile. There is no corresponding case for black pixels, so a white region will compress better than a black region.
On color Macintosh computers, where I have the luxury of a 68020 with an instruction and data cache, I perform additional computation in order to choose from among the following TRLE tile types:
- Solid tiles
- Paletted 1-bit tiles
- Paletted 2-bit tiles
- Raw 8-bit tiles
- RLE encoded tiles
On a color Mac, the encoding process works like this:
- The tile is encoded as a plain RLE tile. During this stage, up to five unique colors from the tile are recorded.
- If the number of unique colors is equal to one, a solid tile is emitted
- If the number of unique colors is equal to two and the size of the RLE encoded tile exceeds that of a paletted 1-bit tile, then a 1-bit paletted tile is emitted
- If the number of unique colors is equal to three or four and the size of the RLE encoded tile exceeds that of a paletted 2-bit tile, then a 2-bit paletted tile is emitted
- If the number of unique colors exceeds fours and the size of the RLE encoded tile exceeds that of a raw 8-bit tile, a raw 8-bit tile is emitted.
The TRLE encoder was written in 68x assembly for best performance.
For the color encoders, a 68020 is assumed and the code makes special
use of 68020 instructions such as bfextu. Most functions take
advantage of as many of available the eight data and eight address
registers as possible, as to minimize memory access.
