AnachronautandClaude Opus 5
3d2ab34229
osFileRead hands over a whole file, which settles anything under 64K and settles nothing above it. CosmOS's own source is above it - the sources together are 104K against 64K of Data Memory - so a machine that is going to assemble itself needs another way to ask. osFileInfo (0d26) says how many blocks a file occupies. osFileBlock (0d27) hands over one of them and says how many of its bytes belong to the file. Between them a program reads a file of any size through a buffer of 256. Blocks rather than bytes from osFileInfo is forced, not chosen: a file on a sixteen megabyte disk is up to twenty four bits long and a pointer holds sixteen. osFileBlock's count answers in DP3 for the same kind of reason - a whole block is 256 bytes, which does not fit in a register, and a count that reported it as zero would make every reader special-case the end. Nothing is kept open. Every call names the file, so there is no handle to leak and nothing left behind by a program that stops halfway. Taken at its word that means searching the directory once per block, so the system remembers where the last file it was asked about lives; every path that can change what a name means calls fileForget, including the shell's own delete and rename, which do not go through the services. Correctness never depends on the cache - a cache thrown away is indistinguishable from one never filled. Measured on a 329 block file: 7% of the run saved when the file is the first directory entry, 11% when it is the sixteenth. These two say WHY when the answer is no, which the others do not. Elsewhere the only useful response to a failure is to give up, so one value suffices. These are asked questions, and running off the end is how a reader learns it has finished, so it gets an answer of its own: 1 no disk, 2 no such file, 3 past the end, 4 the disk refused. Apps/Stream.asm reads an 84,000 byte file through 256 bytes. The check that matters is the second one: a small file read BOTH ways - whole with osFileRead and streamed - with the two checksums compared, so streaming is measured against the path already known to work rather than against a number someone wrote down. The checksum is Fletcher's rather than a sum, because a sum is the same whatever order the bytes arrived in and the order is exactly what streaming has to get right. Both checksums were also confirmed against the same arithmetic run on the host. The rest of the test is the cache: two files read alternately catch a memory that missed the name changing, and a rename catches one that missed the file moving - and that one would otherwise pass, since the blocks are still there holding the same bytes. The test file is generated rather than taken from the repository. The CosmOS sources would be a truer picture and would move the recorded checksum every time a line of CosmOS was edited, putting a real difference in a crowd of meaningless ones - the same trap the cycle counts used to set. cosmosBreak's recorded output moves by two bytes in two pointers: SbfsIndex added two bytes to the filesystem's data and Break prints the system addresses the registers happened to hold. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
142 lines
10 KiB
Markdown
142 lines
10 KiB
Markdown
# SplitBit Emulator
|
|
|
|
## Overview:
|
|
SplitBit is a custom 8 bit system designed for hobbyist projects and experimentation: a CPU with its own instruction set, an interrupt and vector system, a bus that programs can enumerate, and a memory controller that can load code. The SplitBit Emulator is a C implementation of it. It allows users to load and run binary programs created for SplitBit interactively from the command line.
|
|
|
|
### Features:
|
|
- 8-bit Harvard Architecture: The system memory is separated into two 64k banks, one for the Program Memory and another for the Data Memory.
|
|
- Custom ISA: A fully implemented instruction set architecture optimized for simplicity and easy assembly programming.
|
|
- Debug Mode: Single-step through instructions and monitor the CPU's registers as they change through each cycle.
|
|
- CLI Based: Debug messages and CPU input and output are supported through the command line.
|
|
- Binary File Support: Load programs and data from binary files.
|
|
- Modular Codebase: Mostly clean separation of CPU, I/O, and utility functions for easy modification.
|
|
- Interrupts: Software traps, hardware lines from devices, and faults, all arriving through one vector table with a full context save.
|
|
- Devices: A bus registry that says what a machine is made of, so a program can ask rather than being told.
|
|
- Filesystem: SBFS, read and written by SplitBit itself, and by a host tool that speaks the same format so an image can be moved either way.
|
|
- Loadable Programs: A program that was not booted from carries a header saying where it belongs, and Programs/loader.asm reads one off a disk, puts it there, and runs it.
|
|
- An Operating System: CosmOS boots the machine, mounts a disk, lists what is on it, loads a program and runs it, and takes the machine back when it finishes. It comes with a library of programs to run, including a game and a line editor that writes files a person typed.
|
|
- System Services: A loaded program reaches the console and the disk through numbered software interrupts rather than carrying a copy of the code that drives them. The numbers are written down in one file that both sides include, so neither ever types one. It took the editor from 4941 bytes to 1983 without changing a line of what it does.
|
|
- Streaming Reads: A file bigger than the machine's memory is read a block at a time, through services that keep nothing open between calls. CosmOS's own source is 104K against 64K of Data Memory, so this is what a self-hosted assembler will stand on.
|
|
- Storage: A block device with 256 byte blocks and 16 megabytes of them, backed by an image file on the host. It knows blocks and not files, because a filesystem is meant to be software SplitBit runs.
|
|
- Memory Controller: Reads and writes Program Memory, moves blocks between memory banks, reaches memory that devices bring with them, and guards a range against being written by accident. It is how a SplitBit machine loads a program.
|
|
- Assembler: Assemble human readable assembly language files directly into SplitBit compatible binary files. Supports including external files, handling labels, alignment and reservation, and defining Program, Data and Vector segments.
|
|
|
|
### Installation:
|
|
1) Clone the repository:
|
|
```
|
|
git clone https://github.com/RealBusinessAccount/SplitBit-Emulator.git
|
|
cd SplitBit-Emulator
|
|
```
|
|
2) Build the Emulator, the Assembler and the disk tool: You'll need gcc and make or similar.
|
|
```
|
|
make
|
|
```
|
|
The sources are ISO C, and build clean under -std=c11 -pedantic with -Wall -Wextra. Beyond ISO C they need POSIX.1-2008, which the makefile asks for by name, and getopt_long for the long form of the command line options.
|
|
3) Assemble a program:
|
|
```
|
|
./Assembler Programs/hello.asm
|
|
```
|
|
|
|
4) Run the program:
|
|
```
|
|
./SplitBit hello.bin
|
|
```
|
|
|
|
### Usage:
|
|
```
|
|
./SplitBit [options] [binary file]
|
|
```
|
|
#### Options:
|
|
- -d, --debug: Enable debug mode to single step through cycles. Each key press advances one instruction.
|
|
- -c, --cycles N: Stop after N cycles rather than running until the program halts. Useful for programs that never halt, and for getting the same output from a run every time.
|
|
- -f, --fast: Run as fast as the host machine allows, ignoring the emulated cycle rate.
|
|
- -D, --disk \<file\>: Attach a disk image, creating a 128K one if the file is not there.
|
|
- -W, --write-protect: Attach the disk read only. A disk whose image the host will not let you write is read only whether you ask for this or not.
|
|
- -h, --help: Show help and usage information.
|
|
|
|
#### Notes:
|
|
- If the CPU reads a byte that is not an instruction, it goes to the fault handler the program installed. If it installed none, it raises the Fault Flag and halts, and the emulator reports the byte and the address it was found at and exits with a non zero status. The same happens if a program or a device asks for a handler that was never installed.
|
|
|
|
### Usage:
|
|
```
|
|
./SplitDisk <command> <image> [arguments]
|
|
```
|
|
#### Commands:
|
|
- format \<image\> [blocks] [dirblocks]: Lay down a fresh filesystem. 512 blocks and 8 of directory by default, which is 128K and room for 64 files.
|
|
- list \<image\>: Show what is on the disk.
|
|
- put \<image\> \<file\> [name]: Put a host file onto it. Without a name it uses the file's own, which is often longer than the 22 characters a name may be.
|
|
- get \<image\> \<name\> [file]: Take one off it.
|
|
- delete \<image\> \<name\>: Remove one.
|
|
|
|
#### Notes:
|
|
- SplitDisk speaks the same on disk format SplitBit does, so an image it makes is one the machine can read, and one the machine writes is one it can read back. SplitBit writes its own filesystem now, so this is not the only way to get something onto a disk; it is still the only way to get a program onto one, since nothing running on the machine assembles anything yet.
|
|
- Files are laid down contiguously, so a disk can have free blocks without having them in one piece. When that happens put says so rather than putting part of a file on.
|
|
|
|
### Usage:
|
|
```
|
|
./Assembler [options] [assembly file]
|
|
```
|
|
#### Options:
|
|
- -o \<file\>: Write the binary to this path.
|
|
- -I \<dir\>: Look in this directory for included files. May be given more than once.
|
|
- -M \<file\>: Write out which source files the binary depends on, as a make rule.
|
|
- -h, --help: Show help and usage information.
|
|
|
|
#### Notes:
|
|
- Without -o, the assembled binary is saved with the same name as the assembly source file, with a .bin extension, in the directory that you call the assembler from.
|
|
- Included files are looked for beside the file that includes them, and then along the directories given with -I.
|
|
|
|
### Building Programs With Make:
|
|
The assembler is built to work with make. The -o option puts the binary where the build system wants it, and -M writes out which libraries went into it, so that editing a library reassembles everything that includes it.
|
|
|
|
Programs/makefile does this for the programs in this repository:
|
|
```
|
|
cd Programs
|
|
make
|
|
```
|
|
|
|
The rule it uses is small enough to copy into your own projects:
|
|
```
|
|
$(BUILD)/%.bin: %.asm
|
|
@mkdir -p $(@D)
|
|
$(ASM) -I Libraries -M $(@:.bin=.d) -o $@ $<
|
|
|
|
-include $(BINARIES:.bin=.d)
|
|
```
|
|
|
|
### Tests:
|
|
The test suite assembles and runs every program in Programs/ and compares the results against recorded output.
|
|
```
|
|
make test
|
|
```
|
|
Disk images that tests read from are built by Tests/makedisks.sh before the run, using SplitDisk. A test that reads one is therefore checked against a filesystem written by different code from the same written format, rather than against itself.
|
|
|
|
The disk tool is checked separately by Tests/disk.sh, which make test runs afterwards: it puts files of every awkward size onto an image and takes them off again, and checks that the things the format says cannot happen are refused.
|
|
|
|
Tests/terminal.sh checks the things a recorded output cannot see. Every other test pipes input in and output to a file, which answers what a program prints and is blind to two whole classes of behaviour: **when** something is printed, since piped output is buffered and flushed at exit, so a prompt shown before its answer is asked for and one shown an hour late produce identical files; and **what happens to the terminal**, since key mode only touches one when there is one. Both have gone wrong here, and both were found by a person whose terminal stopped working rather than by anything in this suite. So it runs the emulator under a pseudo-terminal and asks the questions directly: that a prompt arrives before input is read, that a keystroke arrives without Return, that the terminal is handed back however the machine dies, and that suspending and resuming leave it as they found it.
|
|
|
|
A cycle count is deliberately **not** part of a recorded result. The last line of the emulator's output has the number taken out of it before anything is compared, keeping only whether the program stopped on its own or ran into its limit, which is behaviour. Two instructions added to CosmOS used to move that number in six unrelated files at once, so a real difference would arrive in a crowd of meaningless ones. Anything that wants to measure cycles should say so in a test of its own.
|
|
|
|
Tests/docs.sh then checks the manuals against the code: that every instruction has a row and every row is an instruction, that the counts in the headings are right, that every directive is written down, that every routine the manuals promise exists, and that the worked examples still assemble to the bytes printed beside them. Documentation goes stale quietly, and this is what stops it.
|
|
|
|
Tests are defined in Tests/manifest, one line per program. To record the current output as the expected result, after you have checked that it is correct:
|
|
```
|
|
make bless
|
|
```
|
|
Programs are built inside Tests/build, so running the suite never overwrites the binaries in Programs/. To run only some of the tests, call the runner directly with their names:
|
|
```
|
|
./Tests/run.sh hello 8bitFibonacci
|
|
```
|
|
|
|
To rebuild both tools with the address and undefined behaviour sanitizers and run the suite under them:
|
|
```
|
|
make sanitize
|
|
```
|
|
This catches reads and writes past the end of an array, use after free, leaks, and undefined arithmetic. It also fills fresh allocations with a junk pattern, which turns a read of uninitialised memory from something that quietly works into something the tests notice. It takes about twice as long as make test, and puts the ordinary binaries back when it finishes.
|
|
|
|
### Additional Info:
|
|
For more information on the custom ISA and programming for SplitBit, see the Programming Manual and Assembler Manual.
|
|
|
|
### License:
|
|
This project is licensed under the Apache License, Version 2.0. You may obtain a copy of the License at [http://www.apache.org/licenses/LICENSE-2.0](http://www.apache.org/licenses/LICENSE-2.0).
|