This is some very early work towards a simple BIOS for the 1802 membership card.
Since this is a side project targeting a particular piece of hardware, it's not super-flexible. I've made lots of assumptions.
We presume that RAM occupies 0x0000-0x7fff and EEPROM occupies
0x8000-0xffff. There are several special-purpose regions.
| Address | Purpose |
|---|---|
0x0000 |
Initial program counter and EEPROM loader target |
0x0100 |
Toggled-in loader target |
0x7fff |
Stack (grows downward) |
0x8000 |
EEPROM loader entrypoint |
0xfb00 |
Start of BIOS |
0xff00 |
BIOS virtual function table |
0xfffc |
BIOS version |
0xfffe |
BIOS XOR checksum |
We use the following register conventions (which might be expanded as the BIOS
function call ecosystem builds out). Registers r2 through r6 are described
in the 1802 programming guide as part of the "Standard call and return
technique" (SCRT), from which we feel no strong urge to deviate.
| Register | Purpose |
|---|---|
r0 |
DMA |
r1 |
ISR |
r2 |
Stack pointer |
r3 |
Program counter |
r4 |
call trampoline program counter |
r5 |
retf trampoline program counter |
r6 |
Return address |
r7 |
Reserved for future use |
r8-rf |
General purpose |
We presume that there is no hardware UART, and so we implement a bit-banged
half-duplex UART in software. EF3 is used for receive, and Q for transmit.
Both signals are presumed to be inverted, from the 1802's point of view. For
example, seq brings the transmit line low, and bn3 branches when the receive
line is high.
We presume that 1802 is clocked with a 4MHz source and uses 8 clock pulses per machine cycle. Most instructions uses 2 machine cycles (4us), except for the long-branch instructions that require 3. While the PLLs themselves are pretty compact, and can easily support 9600 baud (~26 instructions) or higher, we generally prefer 2400 baud (~104 instructions), which provides ample room for function calls and intermediate logic.
This project uses the following toolchain:
- ASM/02: Cross compiler
- cosmac_emu: Emulator, for tests
- pyserial: Serial interfacing
To build stage1.bin and other images, ensure that asm02 is in your path, and run make.
To run tests, enter the test directory, ensure that cosmac_emu is in
your path, and run make test.
The basic idea:
make- Toggle in
loader.binand reset. ./bootstrap.py load --image stage1.bin- Toggle in
lbr 0100h(0c 01 00) and reset. ./bootstrap.py burn
Because the BIOS is not relocatable, we make a stage1 image that directly
includes enough functionality to run the UART and write to EEPROM. Appended to
that image are the loader and BIOS images that we intend to write EEPROM.
The bootstrap script loads the stage1 image into 0x0100 in step 3, and then
uses the minimon interface to write the embedded loader and BIOS images to
EEPROM in step 5. During the burn, the front display of the membership card
should briefly show the 8-bit checksum for the loader image, followed by the
8-bit checksum for the BIOS. The latter should always be 00, since the BIOS
is stamped with its own partial checksum.