bootstrapOS

bootstrapOS

Many OS tutorials start in assembly or C++. We start in machine code.

In this Raspberry Pi 4 tutorial, we'll write a self-hosted assembler in machine code. We'll then evolve that compiler (by implementing new features then using those features in the compiler source code) into something capable of wrangling a high-level language. Finally, in our high-level language, where most tutorials start, we'll write a simple web server to one day host the tutorial's own documentation.

Progress

Milestone: bootstrap machine code ?

• print to qemu uart
• properly setup uart, wait for write, print 'x'
• wait for read, read, print uart input
• write all to memory then dump memory in
• copy mem_in to mem_out then modify above code to print mem_out
• use bytes to indicate length of output
• routine to read a certain number of bits to memory, use it to copy machine code
• remove leading whitespace, ; command for comments
• print raw binary

Milestone: big-endian machine code ?

• print binary in little-endian form
• rewrite codebase in big-endian form (ehh, leave most formatting for later)

Milestone: labeled goto ?

• J: relative branch with binary
• L: label for goto (read into memory tape with scheme pos4 name )
• GOTO: goto label
• rewrite codebase using labeled GOTOs

Milestone: conditional goto ?

• GOEQ
• GONE
• GOGT
• GOLT
• GOGE
• GOLE
• rewrite codebase using conditional goto

Milestone: labeled functions ?

• CALL: "goto" but with linked branching
• rewrite codebase

Milestone: emitting strings

• MOV reg char
• EMIT string: macro for series of MOV reg char and CALL emit with escape sequence support
• rewrite codebase

Milestone: bootstrap assembly

• RET: return after function call
• MOV reg hex
• MOV reg dec
• MOV reg reg
• ADD reg reg [reg/value]: set the value of a register
• SUB reg reg [reg/value]: set the value of a register
• LDB reg reg_mem offset: load a byte
• STB reg reg_mem offset: store a byte
• ORR reg reg (reg >>/<< value)
• AND reg reg (reg >>/<< value)

Milestone: stack-lang functions

• .func [name] [num locals]
• .call [name]

Milestone: push, pop, and dup

• .push constant [val]

• .dup

• .pop local [idx]

• .push local [idx]

• .pop param [idx]

• .push param [idx]

• .pop global [idx]

• .push global [idx]

Milestone: stack-lang memory access

• .load (..., addr) -> (..., val)
• .store (..., val, addr) -> (...)

Milestone: arithmetic

• .add
• .sub

Milestone: heap allocation

• malloc stack function (..., num bytes) -> (..., addr)

Milestone: parsing and compiling

• constants
• unary operations
• binomial operations
• global variable declarations
• basic functions (with goto for fn main)
• function calling and parameters
• local variables
• conditional blocks
• loops
• break from loops

---

• refactor circle library's ethernet driver
• rewrite ethernet driver in our high-level language
• TCP/IP stack
• http server
• serve multiple files
• preemptive multitasking

Lines of code over time

for f in ./os/*.txt; do wc -l $f; done | Rscript bar-gen.R ; feh ./loc-stats.png