FlashOS tour · 12 / 15

12. Full-screen Apps: less & edit

/bin/less and /bin/edit are Rust EL0 programs under userland/. They do not draw through a framebuffer driver. They use terminal control sequences, raw key decoding, and the alternate screen over the same console descriptors as the shell.

Shared userland building blocks

crates/flibc/ contains the reusable cores:

Module	Responsibility
`keys.rs`	decode escape sequences, arrows, control keys, and Tab
`tui.rs`	terminal screen entry/exit and rendering helpers
`pager.rs`	line indexing, viewport position, and scroll clamping
`gapbuf.rs`	editable text storage with a movable gap
`heap.rs`	userland bump allocation over `brk`/`sbrk`

These pure state machines are host-testable without booting the kernel. The program crates connect them to file descriptors and syscalls.

Pager flow

userland/interactive/less/ reads a file into bounded user memory, indexes line starts, enters the alternate screen, renders the visible range, and changes the viewport in response to decoded keys. On exit it restores the normal screen.

Editor flow

userland/interactive/edit/ is the main heap consumer. It loads text into a gap buffer, moves the gap to make insertion and deletion local, renders the viewport, and saves through the current FAT32 mutation surface.

file bytes → gap buffer → edit operations → rendered viewport
                                      ↓
                              unlink/create/write

The save sequence works around the absence of general file truncation. It is appropriate for the current filesystem, but not yet an atomic save protocol.

Terminal ownership

Full-screen tools temporarily own the interactive terminal. They must restore screen state even on ordinary exit. Kernel diagnostics remain on Mini-UART, so they do not depend on the user program's alternate-screen lifecycle.

Relationship to future FlashUI

These programs are current, shipped tools. FlashUI is a later native TUI initiative that will embed FlashShell and eventually become the default post-login session. Its MVP is planned to hand the terminal to foreground programs like edit and restore its own screen afterward, rather than emulate an entire terminal or duplicate those programs.

/bin/fsh, /bin/less, and /bin/edit remain real standalone recovery and foreground paths through that future integration.

[!TIP] The right design boundary is state versus transport: buffer, pager, and renderer logic can be host-tested; raw console I/O and real FAT32 persistence need runtime or hardware evidence.

Next, we examine the layers of evidence that qualify those boundaries.