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diff --git a/docs/internals.md b/docs/internals.md new file mode 100644 index 0000000..c1007d1 --- /dev/null +++ b/docs/internals.md @@ -0,0 +1,239 @@ +# gbos internals + +The deep technical breakdown that used to live in the README. The short story +is in [../README.md](../README.md); this is the long one. + +## Why this shape (the hardware reality) + +- No MMU, no memory protection. "Unix" here = the API/process model, not isolation. +- CPU only ever sees **64 KiB**; RAM is banked. +- **Kernel code lives in ROM** (`$0000-$3FFF`, up to 8 MiB via MBC5) — free, read-only. +- **Program text lives in ROM banks** (`$4000-$7FFF`) — the V7 "pure text" idea. +- **Per-task RW state** (data/bss/heap/stack) lives in one **8 KiB cart-RAM bank** + (`$A000-$BFFF`, MBC5, up to 128 KiB / 16 banks). +- The context switch swaps banks, so it **must** run from **HRAM** (never banked) + and never touch a swappable stack mid-swap. + +## Memory map + +``` +$0000-$3FFF ROM0 kernel core (rst/IRQ vectors, init, sched, syscalls) - fixed +$4000-$7FFF ROMX current task TEXT (read-only program code) +$8000-$9FFF VRAM terminal (BG) + on-screen keyboard (window) +$A000-$BFFF SRAM current task DATA/BSS/HEAP/STACK (MBC5 RAM bank) +$C000-$CFFF WRAM0 kernel globals + kernel stack (never swapped) +$D000-$DFFF WRAMX terminal shadow, FS caches (per-process u-area reserved) +$FF80-$FFEE HRAM context-switch trampoline (111 bytes) +$FFEF-$FFF2 HRAM bank shadows + switch target +``` + +## Process control block (`include/gbos.inc`) + +`STATE, PID, SP, ROMB(16b text bank), RAMB(data bank), WRAMB(u-area), PARENT, EXIT` +— 10 bytes. `MAX_PROCS = 8`. + +## Syscall ABI + +User loads `C` = syscall number, args in `DE`/`B`/`HL`, then `rst $30`. +Return in `A` (and `B` for `wait`). **The trap clobbers `BC`/`DE`/`HL`** (it uses +`HL` to index the dispatch table), so userland must save any pointer it needs +across a syscall (`push hl` / `pop hl`). Libc syscall stubs handle this for C. + +Core calls: `exit fork read write exec wait getpid yield kill` plus console, +filesystem (`open getb putb list remove mkdir chdir`), process +(`ps-info reap setin setout`), time (`uptime sleep`) and networking +(`ssend srecv net` — see below) families. + +### The shell (`usr/sh.c`) + +init `exec`s the shell (written in C). It reads a line, tokenizes it, and runs +commands with **I/O redirection and pipes**: + +``` +$ echo hello > note # redirect stdout to a file +$ cat note +hello +$ wc < note # redirect stdin from a file +1 1 6 +$ cat readme | wc -l # pipe (via a temp file) +2 +``` + +Operators must be space-separated (`cat readme > out`). Command names are +resolved by the kernel (`sys_lookup` + `NameTable`). + +**How I/O routing works.** Each process has a `stdin`/`stdout` fd in its PCB +(default `$FF` = console). `read`/`write`/`putc` route through the kernel +(`KGetc`/`KPutc`): console when the fd is `$FF`, otherwise the filesystem +(`getb`/`putb`). The shell forks a child, the child `setin`/`setout`s to open +files, then `exec`s — so the program's I/O lands in the file. A pipe +`a | b` is run as `a > __pipe ; b < __pipe ; rm __pipe`. + +### Process lifecycle (exit / wait / reaping) + +The classic Unix zombie/reap dance, adapted to banked memory: + +- **`exit(code)`** sets `PS_ZOMBIE` + status, **reparents** any children to init + (pid 1), **wakes** the parent if it's blocked in `wait()`, then schedules + away forever. Its resources are freed by the reaper, not here. +- **`wait()`** scans for a `PS_ZOMBIE` child. Found → **reap**: free its + cart-RAM bank (back to the allocator) and its PCB slot, return pid + code. + Children exist but none dead → set self `PS_BLOCKED` and yield, retry on wake. + No children → return `$FF` (`ECHILD`). +- The scheduler skips non-`READY` procs; `FindNextReady` returns carry when + nothing is runnable so `yield` just keeps the caller running (idle). +- **Cart-RAM banks are a free list** (`wBankUsed` bitmap): `AllocRamBank` / + `FreeRamBank`. Verified by running 6 workers through only 3 child banks + (`w2w3w4w5w6w7`) — each reaped bank is recycled by the next `fork`. + +### exec (`sys_exec`, src/proc.asm + programs.asm) + +Where the text-in-ROM model pays off. Programs are linked to run from the ROMX +window (`$4000-$7FFF`) and stored in their own ROM banks (`programs.asm`). +`exec(B=program id)`: + +1. looks up `ProgramTable[B]` = (ROM bank, entry), +2. sets `PROC_ROMB` and maps the bank live into `$4000-$7FFF`, +3. resets the stack to the top of the task's (already mapped) cart-RAM bank, +4. `jp`s to the entry — it never returns. + +Proof it's really bank-driven: both demo programs are ORG'd at the **same** +address `$4000` in **different** banks (02 and 03). The parent `exec`s +`PROG_PING`, the child `exec`s `PROG_PONG`, and the output `1212...` (2000/2000, +zero garbage) can only happen if each process executes its own bank via +`PROC_ROMB`. + +### fork (`sys_fork`, src/proc.asm) + +The interesting syscall. It enters on the parent's user stack (which lives in +the `$A000-$BFFF` cart-RAM window), so before it can swap that window it +**switches to a kernel stack in WRAM0**. Then it: + +1. allocates a free PCB slot + a fresh cart-RAM bank (bump allocator), +2. copies the parent's whole 8 KiB bank → the child bank, 256 bytes at a time + through a WRAM bounce buffer (only one cart-RAM bank is visible at once), +3. plants a **switch-in frame** in the child at `Uafter-8` (`Uafter` = parent + SP at entry): the copied return address is already there, so it just zeroes + the saved `hl/de/bc/af` slots — the child resumes at the post-`fork` PC with + `A=0`, +4. fills the child PCB (shared ROM text bank, new RAM bank, parent as PPID), +5. restores the parent stack and returns the child pid. + +## The context switch (`src/hram.asm`) + +The crux. `hSwitchTo(DE=&incomingPCB)`: +1. push full register context onto the **outgoing** task's stack +2. save `SP` into the outgoing PCB (WRAM0, always mapped) +3. program incoming banks: **RAM bank → SVBK → ROM text bank** +4. load `SP` from the incoming PCB (its banks are now mapped) +5. pop context, `ret` into the incoming task + +Runs from HRAM so changing `SVBK`/RAM-bank never pulls the rug out from `PC` or +the stack. New tasks are bootstrapped with a fake frame (`ProcSetupStack`) so the +first switch-in `ret`s straight to their entry point. + +## The network stack (`src/socket.asm`, `src/net.asm`) + +The LCD terminal + on-screen keyboard freed the link port from console duty, +so **the link port is the network interface**. The kernel owns everything from +the wire up: byte-level link transfers (`net.asm`), SLIP framing, IPv4 + +checksums, ICMP (inbound echo is auto-answered while anything pumps RX), UDP, +and a small TCP (SYN/ACK state machine, MSS 200, single in-flight segment). +Userland sees one `SYS_NET` syscall: `socket / bind / connect / send / recv / +recv_nb / close` driven by a request block — no program touches SLIP or IP. + +Userland networking is plain C on top of that: `dhcp` (DISCOVER→ACK, then +`net_setip()`), `resolve.h` (dotted-quads + DNS A queries to 1.1.1.1), +`netd` (background RX pump so pings are answered), `ping`, `nslookup`, `wget` +(HTTP GET over kernel TCP), `irc` (a full client: channels, /join /msg /me, +nick colors, flood-safe rendering) and `chat`. + +Bytes that arrive **outside** SLIP frames land in a console ring instead — +that's how the host can type into the shell remotely (`tools/gbtype`). + +On the host side, `tools/gbhub` is a virtual switch/router: it spawns (or +accepts, `tools/gbjoin`) several emulator instances, leases each an IP via +DHCP (10.0.0.2+), routes GB↔GB traffic, floods broadcasts, and NATs the rest +out a TUN device to the real internet. `tools/gateway.py` is the older +single-GB gateway (echo/http/chat/irc bridge modes). + +## Filesystem + +A small **block-based, persistent filesystem** on battery-backed cart RAM +(`src/blk.asm` + `src/fs.asm`). Layout on the 32 KiB "disk" (256-byte blocks): + +``` +block 0 superblock (magic + version) +block 1 bitmaps (free blocks + free inodes) +blocks 2-3 inode table (32 inodes x 16 B: type, size, 8 direct block ptrs) +blocks 4.. data blocks +``` + +- **Inodes + nested directories.** A directory is a file of 16-byte entries + (inode# + name), each carrying `.`/`..`. Files are up to **2 KiB** (8 direct + blocks); **32** inodes, 16 entries per directory. +- **Paths + per-process cwd.** `open`/`mkdir`/`chdir`/`remove`/`ls` take paths + (absolute `/a/b` or relative `a/b`); each process has a cwd inode (`PROC_CWD`, + inherited on fork). Shell builtins: `cd`, and a cwd-aware prompt. +- **Bounce buffer.** Only `read_block`/`write_block` touch cart-RAM banking; + everything else works on WRAM buffers. The bitmap + inode table are cached in + WRAMX; data/dir blocks stream through a one-block cache. +- **Persistent.** Formats on first boot (magic/version check), then survives + power-off via the emulator's `.sav`. + +## The terminal & keyboard (`src/term.asm`, `src/osk.asm`) + +- **40×18 color terminal on the CGB LCD**: a 4×8 font packs two characters per + background tile; tiles are line-bound so scrolling rewrites one tilemap row, + not the screen. ANSI-ish color escapes give per-cell fg/bg (see `usr/ansi.c`, + the IRC client's nick colors). An ASCII shadow of the screen lives in WRAM + (`TERM_BUF`, `$D600`) — which is what `tools/gbdemo` reads to sync demos. +- **On-screen keyboard** on the window layer: SELECT toggles it, d-pad + A + types, START sends the line. The terminal underneath is never disturbed + (window enable is one LCDC bit). + +## Writing programs in C + +gbos programs are written in **C**, compiled with **SDCC** (the `sm83` port): + +- `usr/gbos.h` — the userland API; `usr/libc.s` — syscall stubs that preserve + `BC/DE/HL` around `rst $30`; `usr/libc.c` — helpers (`putu`, `atou`, + `argv_parse`, `hasflag`/`optval`, `msleep`). +- `usr/crt0.s` — entry: `call main`, then `exit(0)`; linked first so `_start` + is the `$4000` entry. +- `usr/build.sh` — compiles with SDCC's **native** toolchain (`sdasgb` + + `sdldgb`), links code at `$4000`, extracts a ROM-bank blob; the Makefile + INCBINs it and registers it in the program + shell command tables + (`src/programs.asm`). + +Arguments: the shell leaves `"cmd\0args\0"` at `$A000`; the child inherits it +through `fork`, `getargs()` returns the raw string, `argv_parse()` tokenizes. + +**Toolchain gotchas found the hard way:** SDCC's `--asm=rgbds` mode mis-orders +instructions (emits `ld [hl],a` before the `ld hl,sp+0` that sets the pointer) +— so we use the native asxxxx path and INCBIN the blob instead. String +literals with embedded control chars are also mangled in rgbds mode; C +programs pass explicit lengths and emit newlines via `nl()`. + +## Bring-up bugs kept as cautionary tales + +- **Stack vs. BSS clear:** the kernel stack (`SP=$D000`) grows down into + `$CxFF`, so zeroing all of WRAM0 wiped the live return address. + `ClearKernelRAM` now clears only `$C000-$CBFF`. +- **Syscall ABI vs. dispatch:** `SyscallTrap` originally used `DE` to index the + jump table, clobbering the `write()` buffer pointer passed in `DE`. Dispatch + now indexes via `A`/`HL` only. +- **TCP field offsets vs. 8-bit adds:** socket-struct accessors do `add SK_x` + as an 8-bit immediate, so any field offset ≥256 silently truncates and lands + *inside* the RX buffer — keep TCP state ahead of `SK_RXBUF`. +- **Console ring vs. burst injection:** one `net_pump` drains a whole serial + burst, so the 64-byte console ring must hold a full injected command line. + +## Roadmap + +- [ ] `exec` refinement: copy `.data` from ROM + zero `.bss` for RW globals +- [ ] more tools (`rev`, `grep`, `tail`), true concurrent pipes +- [ ] Preemptive scheduling: real context save in `TimerISR` → `hSwitchTo` +- [ ] Use the `$D000-$DFFF` u-area (SVBK) for per-process kernel state / kstack +- [ ] `brk`/heap allocator inside the task bank (heap up, stack down) +- [ ] Swap whole tasks to cart flash when RAM banks are exhausted (UZI-style) |