VAXstation 100 firmware notes

Some random notes from running the DEC-supplied standard firmware for the VAXstation 100 in an emulator.

The firmware analyzed announces itself as version 05.00. It deviates in minor details from the information in the 1983 "VS100 Engineering Specification".

Memory map

Firmware 5.00 extends the LOOPBACK region to one more word at 4A0004. Apparently this loops back the value written to 480004.

The BBA scratchpad has been moved to 30000. An accesses to a byte at 280000 seems to clear a BBA interrupt.

RAM memory map

Most of the RAM is available for uploaded software. However, regions at the ends are used by the ROM for fixed purposes:

000000	32 longwords	Exception vectors
000080	15 longwords	Saved screen contents under mouse pointer
0000C0	two words	Mouse x,y copied from hardware register
0000C4	two words	Old mouse x,y screen coordinates
0000C8	two words	New mouse x,y screen coordinates
<01FFC0	extends towards lower addresses	Stack
01FFC0	one longword	Keyboard fifo read pointer
01FFC4	one longword	Keyboard fifo write pointer
01FFC8	one longword	Tablet fifo read pointer
01FFCC	one longword	Tablet fifo write pointer
01FFD0	one word	Cursor column
01FFD2	one word	Cursor line
01FFD4	Connection timeout counter
01FFD6	16 bytes	Keyboard output fifo
01FFE6	16 bytes	Tablet output fifo
01FFF6	one byte	Size data in keyboard fifo
01FFF7	one byte	Size data in tablet fifo
01FFF8	one byte	Mouse buttons
01FFF9	one byte	Keyboard input (LK201 code)
01FFFA	one byte	Tablet input
01FFFB	one byte	State of CTRL and SHIFT, for entering maintenance menu

Error LEDs

Indicates which test failed. During power-up, green light turned off. During idle loop, green light remains on.

Apparently there's a bug.

Interrupts

All VS100 interrupts use the autovector feature.

Firmware 5.00 doesn't use the level 1 interrupt. The firmware only reads the mouse position during maintenance test.

Keyboard test

The firmware transmits code FD (hex) to the LK201 keyboard, which commands it to jump to the power-up sequence and self-test. The keyboard transmits a four-byte message back. The VS100 ignores the keyboard ID, and checks that the third is not 3D.

This information is from the VT220 Technical Manual.

D0 flags

The firmware maintains a set of flags in register D0.

Bit (decimal)	Meaning
16	Manufacturing mode.
17	BBA present.
18	Link down.
19	Power-up error.
20	LEDs locked.
21	Suppress printing test result.
22	In maintenance menu.
23	Keyboard ok.
24	BBA done.
25	Vertical retrace interrupt.
26	Test not executed. (E.g. hardware not present.)
27	Test timed out.
28	Host wants attention. (Same vector as TRAP #1?!?)

Built-in graphics

The firmware has a mouse pointer, a text font, and three icons. The first two are straight bitmaps, whereas the icons are run length encoded.

This is the font.

This icon is displayed when a user can be logged into the system.

This icon is displayed when there's an internal error.

This icon is displayed when there's a communication problem.

Communication

The firmware communicates with the host through a set of eight 16-bit registers. (There is also a 256K window into the host Unibus address space.)

These registers are written.

CSR0	Bit 0 - clear "go" bit.
CSR1	Interrupt reason.
CSR2	Peripheral event.
CSR3+CSR4	Unibus window address.
CSR5	Firmware version.
CSR6	BBA info.

These events are sent. Numbers are hexadecimal bit masks.

0000
0001	Init done.
0002	Function performed ok.
0008	Mouse buttons.
0080	Powerup done.
8000	Function error.
8001	Bad function number.
8002	Bad command number.

These registers are read.

CSR0	Bit 0 - check "go" bit. Bits 1-5: function code.
CSR3+CSR4	Parameter.

These functions are implemented in ROM. Numbers are decimal.

1	Init - setup hardware.
2	Command - further decoded. Parameter is an address in Unibus memory.
3	Start firmware. Parameter is address.
5	Reset.
18	Clear infinite retry counter.
19	Clear finite retry counter.

BitBlit Accelerator

The BBA is a custom microcoded processor based on the Am2901 family. It was initially to be an optional add on, but later it was required. The BBA can access the frame buffer and has its own 256 by 16 bit scratchpad RAM.

The 68000 CPU communicates with the BBA through the scratchpad. When a command has been written, the CPU pokes a hardware register to signal the BBA to start. The BBA interrupts then CPU when done.

The contents and structure of the commands are not yet known. The first word in the scratchpad is apparently a bit mask indicating various functions to be done. Data follows.