actually maybe let's not just yet

at throug reference implementation

.asm-lsp.toml

@@ -0,0 +1,21 @@
+[default_config]
+version = "0.10.0"
+assembler = "nasm"
+instruction_set = "x86/x86-64"
+
+[default_config.opts]
+compiler = "zig"
+compile_flags_txt = [
+  "cc",
+  "-x",
+  "assembler-with-cpp",
+  "-g",
+  "-Wall",
+  "-Wextra",
+  "-pedantic",
+  "-pedantic-errors",
+  "-std=c2y",
+  "-mllvm --x86-asm-syntax=intel",
+]
+diagnostics = true
+default_diagnostics = false

.gitignore

@@ -1,2 +1,3 @@
 zig-out
 .zig-cache
+.kate-swp

README.md

@@ -0,0 +1,33 @@
+an operating system made in nasm assembly and (in the future) zig, using the zig build system.
+this project is based on [this video series](https://www.youtube.com/playlist?list=PLFjM7v6KGMpiH2G-kT781ByCNC_0pKpPN) by [nanobyte](https://www.youtube.com/@nanobyte-dev)
+
+# HOWTO
+
+## dependencies
+
+this project requires the nasm assembler, mtools for floppy disk generation and zig to build.
+this project uses qemu as the default emulator, but this is optional
+
+## building
+
+```bash
+git clone https://dario48.site/git/Dario48/zos.git
+cd zos
+zig build
+```
+
+## running
+
+- with the default emulator
+
+```bash
+cd zos
+zig build run
+```
+
+- with a different emulator
+
+```bash
+cd zos
+emulator_cmd zig-out/bin/main_floppy.img
+```

as386.sed

@@ -0,0 +1,196 @@
+#\
+#       @(#)as386.sed	1.1 - 86/11/17\
+#\
+# This is a sed script which converts Intel 386 assembly code to Unix\
+# 386 assembly code\
+#\
+# This script does not attempt to convert 100% of the ASM386 source code,\
+# it cannot handle the following constructs:\
+#\
+# - strange segmentation schemes\
+# - data declarations beyond the simple db/dw/dd with simple init list\
+# - ascii strings\
+# - structure template addressing (i.e., [ebp].foo)\
+# - complex expressions (parenthesis and operators other than +-*/)\
+# - immediate operands that are not simple constants\
+# - immediate operands with automatically typed memory operands\
+# - source files in upper case\
+# - source files with continued lines\
+#\
+# A typical way to use this sed script is:\
+# tr "[A-Z]" "[a-z]" <infile | sed -f this-script >outfile\
+#\
+#\
+# Meaning of labels:\
+#\
+# cmpress	preserve comments, insert tabs, change '?' to '_'\
+# control	convert '$' control lines\
+# segment	convert segmentation directives\
+# equate	convert 'equ' directives\
+# data		convert db/dw/dd data declarations\
+# modifer	delete address/type modifiers\
+# registr	convert register names\
+# bincons	convert binary/octal/hex conatants\
+# address	regularize addess expressions, delete unneeded tabs\
+# normliz	normalize instruction formats (name-tab-opcode-tab-operands)\
+# operand	swap operands, convert scale/index/base address format\
+# opcode	append b/w to opcode for byte/word register operands\
+# comment	restore preserved comments\
+
+:cmpress
+h
+s/;.*$//
+s/[ 	]*$/	/
+s/[ 	][ 	]*/	/g
+s/\([][)(.,:*/+-]\)/	\1	/g
+s/	?	/	0	/g
+s/?/_/g
+s/[ 	][ 	]*/	/g
+
+:control
+s/^\$[	]*include	(	\([^	.)]*\)[^)]*)/#include	"\1.h"/
+s/^\$[	]*eject	/;	/
+/^\$/ s/^/;/
+
+:segment
+/	segment	e[or]	/ s/^/	.text	;/
+/	segment	ro	/ s/^/	.text	;/
+/	segment	rw	/ s/^/	.data	;/
+/	ends	/ s/^/;/
+/	stackseg	/ s/^/;/
+s/^[	]*name	\([^	]*\)/	.file	"\1"/
+/^[	]*end	/ s/^/;/
+/^[	]*assume	/ s/^/;/
+/^[	]*extrn	/ s/^/;/
+/^[	]*public	[^	,]*	,	/ {
+s/^[	]*public	/	.globl	/
+s/	,	.*$//
+G
+s/\n\([ 	]*\)public\([ 	]\)[^,]*,/\
+\1public\2/
+P
+D
+}
+s/^[	]*public	/	.globl	/
+s/^[	]*comm	/	.comm	/
+s/^\([^	][^	]*\)	proc	/\1	:	;proc	/
+/	endp	/ s/^/;/
+s/^\([^	][^	]*\)	label	/\1	:	;label	/
+s/^[	]*even	/	.even	/
+
+:equate
+s/^\([^	]*\)	equ	\(.*\)	$/#define	:	\1	\2	/
+
+:data
+s/^[	]*db	/	.byte	/
+s/^\([^	][^	]*\)	db	/\1	:	.byte	/
+s/^[	]*dw	/	.value	/
+s/^\([^	][^	]*\)	dw	/\1	:	.value	/
+s/^[	]*dd	/	.long	/
+s/^\([^	][^	]*\)	dd	/\1	:	.long	/
+/^[	]*dp	/ {
+s/^[	]*dp	/	.value	/
+s/$/\/selector of dp pointer/
+G
+s/\n\([ 	]*\)dp\([ 	]\)/\
+\1dd\2/
+P
+D
+}
+/^\([^	][^	]*\)	dp	/ {
+s/^\([^	][^	]*\)	dp	/\1:	.value	/
+s/$/\/selector of dp pointer/
+G
+s/\n[^ 	]*\([ 	]*\)dp\([ 	]\)/\
+\1dd\2/
+P
+D
+}
+s/^\([^	][^	]*\)	struc	/\1	:	;struc	/
+s/^\([^	][^	]*\)	record	/\1	:	;record	/
+
+:modifer
+s/	:	near	/	/g
+s/	:	far	/	/g
+s/	:	byte	/	/g
+s/	:	word	/	/g
+s/	:	dword	/	/g
+s/	short	/	/g
+s/	offset	/	$/g
+/	byte	ptr	/ s/$/?%al?/
+s/	byte	ptr	/	/g
+/	word	ptr	/ s/$/?%ax?/
+s/	word	ptr	/	/g
+s/	dword	ptr	/	/g
+s/	pword	ptr	/	/g
+
+:registr
+s/	e\([abcd]\)x	/	%e\1x	/g
+s/	\([abcd]\)\([hlx]\)	/	%\1\2	/g
+s/	e\([ds]\)i	/	%e\1i	/g
+s/	\([ds]\)i	/	%\1i	/g
+s/	e\([bs]\)p	/	%e\1p	/g
+s/	\([bs]\)p	/	%\1p	/g
+s/	\([cdefgs]\)s	/	%\1s	/g
+
+:bincons
+s/	\([01][01]*\)b	/	0b\1	/g
+s/	\([0-7][0-7]*\)[oq]	/	0\1	/g
+s/	\([0-9][0-9a-f]*\)h	/	0x\1	/g
+
+t address
+:address
+s/\[/+/g
+/;/ !s/	]	\.	/	+	/g
+s/	]//g
+s/	%\([^	]*\)	\*	\([248]\)	+	\([^	%][^	]*\)/	\3	+	%\1	*	\2/g
+s/	%\([^	]*\)	+	\([^	%][^	]*\)/	\2	+	%\1/g
+s/	%\([^	]*\)	\*	\([248]\)	-	\([^	%][^	]*\)/	-	\3	+	%\1	*	\2/g
+s/	%\([^	]*\)	-	\([^	%][^	]*\)/	-	\2	+	%\1/g
+s/	+	-	/	-	/g
+t address
+s/	\([)(,*/+-]\)/\1/g
+s/\([)(,*/+-]\)	/\1/g
+s/	:/:/g
+s/%\([cdefgs]\)s:	/%\1s:/g
+
+:normliz
+/:	/ !s/^\([^	;#][^	]*\)/	\1/
+/:	/ !s/^	\([^	+,-]*\)\([+-]\)/	\1	\2/g
+/:	/ !s/^	\([^	+,]*\)+%/	\1	+%/g
+/:	/ s/^\([^	]*\)	\([^	+,-]*\)\([+-]\)/\1	\2	\3/g
+/:	/ s/^\([^	]*\)	\([^	+,]*\)+%/\1	\2	+%/g
+s/+%\([^	,]*\)/(%\1)/g
+s/	+/	/g
+s/\([:,]\)+/\1/g
+
+:operand
+/[.;#]/ !s/^	\([^	]*\)	\([^,]*\),\([^	]*\)/	\1	\3,\2/
+/[.;#]/ !s/^\([^	][^	]*\)	\([^	]*\)	\([^,]*\),\([^	]*\)/\1	\2	\4,\3/
+/[.;#]/ !s/^	\([^	]*\)	\([0-9][0-9a-fx]*\)\([	,]\)/	\1	$\2\3/
+/[.;#]/ !s/^\([^	][^	]*\)	\([^	]*\)	\([0-9][0-9a-fx]*\)\([	,]\)/\1	\2	$\3\4/
+s/(%\([^)+*]*\)+%\([^)*]*\)\*\([248]\))/(%\1,%\2,\3)/g
+s/(%\([^)+*]*\)\*\([248]\)+%\([^)]*\))/(%\3,%\1,\2)/g
+s/(%\([^)+*]*\)+%\([^)]*\))/(%\1,%\2)/g
+s/(%\([^)+*]*\)\*\([248]\))/(,%\1,\2)/g
+
+:opcode
+/[	,]%[abcd][hl]/ s/$/?%al?/
+/[	,]%[abcd]x/ s/$/?%ax?/
+/[	,]%[ds]i/ s/$/?%ax?/
+/[	,]%[bs]p/ s/$/?%ax?/
+/?%al?/ s/^\([^	]*\)	\([^	]*\)	/\1	\2b	/
+/?%ax?/ s/^\([^	]*\)	\([^	]*\)	/\1	\2w	/
+s/^#define:/#define/
+s/?%a[xl]?//g
+
+:comment
+s/	*$//
+x
+/;/ !s/^.*$//
+s/\([ 	]*\);/;\1;/
+s/^[^;]*;//
+x
+G
+s/\n//
+s/;/\//

build.zig

@@ -1,42 +1,109 @@
 const std = @import("std");
+const LazyPath = std.Build.LazyPath;
 
 // Although this function looks imperative, note that its job is to
 // declaratively construct a build graph that will be executed by an external
 // runner.
 pub fn build(b: *std.Build) void {
-    const initramfs = addNasmFiles(b, AddNasmFilesOptions{
-        .filename = "src/initramfs.asm",
-        .outputname = "initramfs.bin",
-        .addAsObjFile = false,
+    const bootloader = addNasmFiles(b, AddNasmFilesOptions{
+        .filename = "src/bootloader.asm",
+        .outputname = "bootloader.bin",
+        .flags = &.{"-f bin"},
+    });
+    const kernel = addNasmFiles(b, AddNasmFilesOptions{
+        .filename = "src/kernel.asm",
+        .outputname = "kernel.bin",
         .flags = &.{"-f bin"},
     });
 
-    const initramfs_install = b.addInstallBinFile(initramfs.obj, "initramfs.img");
-    initramfs_install.step.dependOn(&truncate(b, initramfs).step);
+    const target = b.standardTargetOptions(.{
+        .default_target = .{
+            .os_tag = .uefi,
+            .cpu_arch = .x86,
+        },
+    });
 
-    b.getInstallStep().dependOn(&initramfs_install.step);
+    const floppy = createFloppy(b, "main_floppy.img");
 
-    const run = b.addSystemCommand(&.{ "qemu-system-i386", "-fda" });
-    run.addFileArg(b.path("zig-out/bin/initramfs.img"));
+    var installToFloppy = installImgToFloppy(b, floppy, bootloader, &.{
+        NamedFile{ .file = kernel, .name = "::kernel.bin" },
+        NamedFile{ .file = b.path("src/reference_implementations/test.txt"), .name = "::test.txt" },
+    });
+
+    const img_install = b.addInstallBinFile(floppy, "main_floppy.img");
+    img_install.step.dependOn(&installToFloppy.step);
+
+    b.getInstallStep().dependOn(&img_install.step);
+
+    const fat_reference_mod = b.createModule(.{
+        .root_source_file = b.path("src/reference_implementations/fat.zig"),
+        .target = target,
+        .link_libc = true,
+    });
+
+    const run = b.addSystemCommand(&.{ "qemu-system-i386", "-fda", "zig-out/bin/main_floppy.img" });
 
     const run_step = b.step("run", "run the os in qemu");
     run_step.dependOn(&run.step);
+
+    const fat_reference = b.addExecutable(.{
+        .root_module = fat_reference_mod,
+        .name = "fat_reference_implementation",
+        .use_lld = false,
+        .use_llvm = false,
+    });
+
+    const fat_install = b.addInstallArtifact(fat_reference, .{});
+
+    const build_references = b.step("references", "build the reference implementations");
+    build_references.dependOn(&fat_install.step);
 }
 
 const AddNasmFilesOptions = struct {
     filename: []const u8,
     outputname: ?[]const u8 = null,
-    addAsObjFile: bool = true,
     flags: []const []const u8 = &.{},
 };
 
-const NasmFile = struct {
-    run: *std.Build.Step.Run,
-    obj: std.Build.LazyPath,
+const NamedFile = struct {
+    file: std.Build.LazyPath,
+    name: []const u8,
 };
 
+fn createFloppy(b: *std.Build, name: []const u8) LazyPath {
+    const dd = b.addSystemCommand(&.{ "dd", "if=/dev/zero" });
+    const Floppy = dd.addPrefixedOutputFileArg("of=", name);
+    dd.has_side_effects = true;
+    dd.addArgs(&.{ "bs=512", "count=2880" });
+
+    return Floppy;
+}
+
+fn installImgToFloppy(b: *std.Build, floppy: LazyPath, bootloader: LazyPath, additional_files: []const NamedFile) *std.Build.Step.Run {
+    const mkfs = b.addSystemCommand(&.{ "mkfs.fat", "-F12", "-nNBOS" });
+    mkfs.addFileArg(floppy);
+    mkfs.has_side_effects = true;
+
+    const dd = b.addSystemCommand(&.{"dd"});
+    dd.addPrefixedFileArg("if=", bootloader);
+    dd.addPrefixedFileArg("of=", floppy);
+    dd.addArg("conv=notrunc");
+    dd.step.dependOn(&mkfs.step);
+
+    var last_step = dd;
+    for (additional_files) |f| {
+        const mcopy = b.addSystemCommand(&.{"mcopy"});
+        mcopy.addPrefixedFileArg("-i", floppy);
+        mcopy.addFileArg(f.file);
+        mcopy.addArg(f.name);
+        mcopy.step.dependOn(&last_step.step);
+        last_step = mcopy;
+    }
+    return last_step;
+}
+
 // adapted from https://codeberg.org/raddari/zig-nasm-lib.git
-fn addNasmFiles(b: *std.Build, options: AddNasmFilesOptions) NasmFile {
+fn addNasmFiles(b: *std.Build, options: AddNasmFilesOptions) LazyPath {
     std.debug.assert(!std.fs.path.isAbsolute(options.filename));
     const src_file = b.path(options.filename);
     const output = options.outputname orelse b.fmt("{s}.o", .{std.mem.sliceTo(options.filename, '.')});
@@ -46,18 +113,7 @@ fn addNasmFiles(b: *std.Build, options: AddNasmFilesOptions) NasmFile {
     nasm.addPrefixedDirectoryArg("-i", b.path("src"));
     const obj = nasm.addPrefixedOutputFileArg("-o", output);
     nasm.addFileArg(src_file);
+    nasm.has_side_effects = true;
 
-    return .{
-        .run = nasm,
-        .obj = obj,
-    };
-}
-
-fn truncate(b: *std.Build, bin: NasmFile) *std.Build.Step.Run {
-    const exec = b.addSystemCommand(&.{"truncate"});
-    exec.addArgs(&.{ "-s", "1440k" });
-    exec.addFileArg(bin.obj);
-    exec.step.dependOn(&bin.run.step);
-
-    return exec;
+    return obj;
 }

build.zig.zon

@@ -28,7 +28,7 @@
 
     // Tracks the earliest Zig version that the package considers to be a
     // supported use case.
-    .minimum_zig_version = "0.14.1",
+    .minimum_zig_version = "0.15.0",
 
     // This field is optional.
     // Each dependency must either provide a `url` and `hash`, or a `path`.

src/bootloader.asm

@@ -0,0 +1,379 @@
+; vim: ft=nasm
+org 0x7C00
+bits 16
+
+
+%define ENDL 0x0D, 0x0A
+
+
+;
+; FAT12 Header
+;
+jmp short start
+nop
+
+bdb_oem:					db "mkfs.fat"			; 8 bytes
+bdb_bytes_per_sector:		dw 512                                                        
+bdb_sectors_per_cluster:	db 1                                                          
+bdb_reserved_sectors:		dw 1                                                          
+bdb_fat_count:				db 2                                                          
+bdb_dir_entries_count:		dw 0E0h                                                       
+bdb_total_sectors:			dw 2880                 ; 2880 * 512 = 1.44MB
+bdb_media_descriptor_type:	db 0F0h                 ; F0 = 3.5" floppy disk
+bdb_sectors_per_fat:		dw 9                    ; 9 sectors/fat
+bdb_sectors_per_track:		dw 18                                                         
+bdb_heads:					dw 2                                                          
+bdb_hidden_sectors:			dd 0                                                          
+bdb_large_sector_count:		dd 0                                                          
+                                                                                          
+; Extended boot record                                                                    
+ebr_drive_number:			db 0                    ; 0x00 floppy, 0x80 hdd, useless
+							db 0                    ; reserved
+ebr_signature:				db 29h                                                        
+ebr_volume_id:				db 68h, 6Fh, 6Dh, 6Fh   ; serial number, value doesn't matter
+ebr_volume_label:			db 'zos    disk'        ; 11 bytes, padded with spaces
+ebr_system_id:				db 'FAT12   '           ; 8 bytes
+
+
+
+
+start:
+	; setup data segments
+	mov ax, 0		; use ax as and intermediary as we can't write to es/ds directly
+	mov ds, ax
+	mov es, ax
+
+	; setup stack
+	mov ss, ax
+	mov sp, 0x7C00	; stack grows downward from where we are loaded in memory
+
+	; some BIOS start us at 07C00:0000 instead of 0000:07C0, make sure we are in the right place
+
+	push es
+	push word .after
+	retf
+
+.after:
+
+
+
+	mov [ebr_drive_number], dl
+
+
+	mov si, msg_loading
+	call echo
+
+
+	; read drive parameters (you never know)
+	push es
+	mov ah, 08h
+	int 13h
+	jc floppy_error
+	pop es
+
+	and cl, 0x3F							; remove top 2 bites
+	xor ch, ch
+	mov [bdb_sectors_per_track], cx			; sectors count
+
+	inc dh
+	mov [bdb_heads], dh						; head count
+
+	; read FAT root directory
+	; compute LBA of root directory = reserved + fats * sectors_per_fat
+	mov ax, [bdb_sectors_per_fat]			
+	mov bl, [bdb_fat_count]	
+	xor bh, bh
+	mul bx									; ax = fats * sectors_per_fat
+	add ax, [bdb_reserved_sectors]			; ax = LBA of root dir
+	push ax
+
+	; compute size of root directory = (32 * number_of_entries) / bytes_per_sector
+	mov ax, [bdb_dir_entries_count]			 
+	shl ax, 5								; ax *= 32
+	xor dx, dx
+	div word [bdb_bytes_per_sector]			; number of sectors we need to read
+
+	test dx, dx								; if != 0 add 1
+	jz .root_dir_after						
+	inc ax									; divistion reminder != 0, add 1
+
+.root_dir_after:
+
+	; read root dir
+	mov cl, al								; cl = number of sectors to read = size of root directory
+	pop ax									; ax = LBA of root directory
+	mov dl, [ebr_drive_number]				; dl = drive number (as saved previously)
+	mov bx, buffer							; es:bx = buffer
+	call disk_read
+
+	; search for kernel.bin
+	xor bx, bx
+	mov di, buffer
+
+.search_kernel:
+	mov si, file_kernel_bin
+	mov cx, 11								; compare up to 11 characters
+	push di
+	repe cmpsb								; repe: repeat until cx = 0
+											; cmpsb: compare ds:si to es:di, they are also incremented or decremented based on a flag
+	pop di
+	je .found_kernel
+
+	add di, 32
+	inc bx
+	cmp bx, [bdb_dir_entries_count]
+	jl .search_kernel
+
+	; kernel not found
+	jmp kernel_not_found_error
+
+.found_kernel:
+
+	; di should point to the adress of the entry
+	mov ax, [di + 26]						; first logical cluster (offset 26)
+	mov [kernel_cluster], ax
+
+	; load FAT from disk into memory
+	mov ax, [bdb_reserved_sectors]
+	mov bx, buffer
+	mov cl, [bdb_sectors_per_fat]
+	mov dl, [ebr_drive_number]
+	call disk_read
+
+	; read kernel and process FAT chain
+	mov bx, KERNEL_LOAD_SEGMENT
+	mov es, bx
+	mov bx, KERNEL_LOAD_OFFSET
+
+.load_kernel_loop:
+
+	; read next cluster
+	mov ax, [kernel_cluster]
+
+
+	add ax, 31 ; tmp hardcoding				; first cluster = (kernel_cluster - 2) * sectors_per_cluster + start_sector
+											; start_sector = reserved + fats + root_directory_size = 1 + 18 + 134 = 33
+	mov cl, 1
+	mov dl, [ebr_drive_number]
+	call disk_read
+
+	; can overflow, risky!!!!
+	add bx, [bdb_bytes_per_sector]
+
+	; cumpute location of next cluster
+	mov ax, [kernel_cluster]
+	mov cx, 3
+	mul cx
+	mov cx, 2
+	div cx									; ax = index of entry in FAT, dx = cluster mod 2
+
+	mov si, buffer
+	add si, ax
+	mov ax, [ds:si]							; read entry from FAT table at index ax
+
+	or dx, dx
+	jz .even
+
+.odd:
+	shr ax, 4
+	jmp .next_cluster_after
+
+.even:
+	and ax, 0x0FFF
+
+.next_cluster_after:
+	cmp ax, 0x0FF8							; end of chain
+	jae .read_finish
+	
+	mov [kernel_cluster], ax
+	jmp .load_kernel_loop
+
+.read_finish:
+
+	; jump to kernel
+	mov dl, [ebr_drive_number]				; boot device in dl
+
+	mov ax, KERNEL_LOAD_SEGMENT				; set segment registers
+	mov ds, ax
+	mov es, ax
+
+	jmp KERNEL_LOAD_SEGMENT:KERNEL_LOAD_OFFSET
+
+	jmp wait_key_and_reboot					; should never happen
+
+	cli
+	hlt
+
+; errors
+
+
+
+
+floppy_error:
+	mov si, msg_read_failed
+	call echo
+	jmp wait_key_and_reboot
+
+kernel_not_found_error:
+	mov si, msg_kernel_not_found
+	call echo
+	jmp wait_key_and_reboot
+
+wait_key_and_reboot:
+	mov ah, 0
+	int 16h									; wait for keypress
+	jmp 0FFFFh:0							; jump to start of bios, should reboot
+
+.halt:
+	cli										; disable interrupts, this way we shouldn't be able to get out of halt
+	hlt
+
+
+; 
+; echo:
+; print something to the screen
+;  - ds:si points to string
+;
+echo:
+	; save the registers we want to modify
+	push si
+	push ax
+	push bx
+
+.loop:
+	lodsb			; load byte from ds:si to al
+	or al, al		; check if next char is null
+	jz .done
+
+	mov ah, 0x0E
+	mov bh, 0
+	int 0x10
+
+	jmp .loop
+
+.done:
+	pop bx
+	pop ax
+	pop si
+	ret
+
+
+
+
+;
+; args: 
+;	ax = lba adress
+; return:
+; 	cx[0..5] = sector number
+; 	cx[6-15] = cilinder number
+; 	dh: head
+;
+
+
+
+lba_to_chs:
+
+	push ax
+	push dx
+
+	xor dx, dx								; clear dw
+	div word [bdb_sectors_per_track]		; ax = LBA / SectorsPerTrack
+											; dx = LBA % SectorsPerTrack
+
+	inc dx
+	mov cx, dx								; cx = sector
+
+	xor dx, dx								; clear dx
+	div word [bdb_heads]					; ax = (LBA / SectorsPerTrack) / Heads = cylinder
+	                                        ; dx = (LBA / SectorsPerTrack) % Heads = head
+	mov dh, dl
+	mov ch, al
+	shl ah, 6
+	or cl, ah								; put upper 2 bits of cylinder in CL
+
+	pop ax
+	mov dl, al
+	pop ax
+	ret
+
+
+;
+; args:
+;	ax = LBA adress
+;	cl = number of sectors to read
+;	dl = drive number
+;	es:bx = pointer to where to store the data
+;
+
+disk_read:
+
+	push ax
+	push bx
+	push cx
+	push dx
+	push di
+
+	push cx									; cx will be overridden
+	call lba_to_chs
+	pop ax									; al = number of sectors to read
+
+	mov ah, 02h
+	mov di, 3								; hoe many times to try
+
+.retry:
+	pusha
+	stc										; set carry flag
+	int 13h									; if no carry flag => it work
+	jnc .done
+
+	; read fail
+	popa
+	call disk_reset
+
+	dec di
+	test di, di
+	jnz .retry
+
+.fail:
+	; all attempts exausted
+	jmp floppy_error
+
+.done:
+	popa
+	
+	pop di
+	pop dx
+	pop cx
+	pop bx
+	pop ax
+	ret
+
+
+
+; arg: dl = drive number
+
+
+
+disk_reset:
+	pusha
+	mov ah, 0
+	stc										; set carry
+	int 13h	
+	jc floppy_error
+	popa
+	ret
+
+msg_loading: 								db 'starting zos', ENDL, 0
+msg_read_failed:							db 'error on floppy read', ENDL, 0
+msg_kernel_not_found:						db 'kernel.bin not found', ENDL, 0
+file_kernel_bin:							db 'KERNEL  BIN'
+kernel_cluster: 							dw 0
+
+KERNEL_LOAD_SEGMENT							equ 0x2000
+KERNEL_LOAD_OFFSET							equ 0
+
+
+times 510-($-$$) db 0
+dw 0AA55h
+
+buffer: 

src/kernel.asm

@@ -1,11 +1,18 @@
 ; vim: ft=nasm
-org 0x7C00
+org 0x0
 bits 16
 
+
 %define ENDL 0x0D, 0x0A
 
+
 start:
-	jmp main
+	mov si, msg_init
+	call echo
+
+.halt:
+	cli
+	hlt
 
 ; 
 ; echo:
@@ -18,15 +25,15 @@ echo:
 	push ax
 	push bx
 
-	mov ah, 0xe
-	mov bh, 0
-
 .loop:
 	lodsb			; load byte from ds:si to al
 	or al, al		; check if next char is null
 	jz .done
 
+	mov ah, 0x0E
+	mov bh, 0
 	int 0x10
+
 	jmp .loop
 
 .done:
@@ -35,28 +42,4 @@ echo:
 	pop si
 	ret
 
-main:
-
-	; setup data segments
-	; use ax as and intermediary as we can't write to es/ds directly
-	mov ax, 0
-	mov ds, ax
-	mov es, ax
-
-	; setup stack
-	mov ss, ax
-	mov sp, 0x7C00	; stack grows downward from where we are loaded in memory
-
-	; print the hello world
-	mov si, msg_hello
-	call echo
-
-	hlt
-
-.halt:
-	jmp .halt
-
-msg_hello: db 'Hello, world!', ENDL, 0
-
-times 510-($-$$) db 0
-dw 0AA55h
+msg_init: 								db 'started kernel', ENDL, 0

src/reference_implementations/fat.zig

@@ -0,0 +1,173 @@
+const std = @import("std");
+
+const Errors = error{ InvalidSyntax, ReadSectors, FileNotFound };
+
+var buffer: [1024]u8 = undefined;
+
+var g_Fat: []u8 = undefined;
+
+pub const std_options: std.Options = .{
+    .log_level = .info,
+};
+
+const BootSector = extern struct {
+    BootJumpInstruction: [3]u8 align(1),
+    OemIdentifier: [8]u8 align(1),
+    BytesPerSector: u16 align(1),
+    SectorsPerCluster: u8 align(1),
+    ReservedSectors: u16 align(1),
+    FatCount: u8 align(1),
+    DirEntryCount: u16 align(1),
+    TotalSectors: u16 align(1),
+    MediaDescriptorType: u8 align(1),
+    SectorsPerFat: u16 align(1),
+    SectorsPerTrack: u16 align(1),
+    Heads: u16 align(1),
+    HiddenSectors: u32 align(1),
+    LargeSectorCount: u32 align(1),
+
+    // ebr
+    DriveNumber: u8 align(1),
+    _Reserved: u8 align(1),
+    Signature: u8 align(1),
+    VolumeId: u32 align(1),
+    VolumeLaber: [11]u8 align(1),
+    SystemId: [8]u8 align(1),
+};
+
+var g_BootSector: BootSector = undefined;
+
+const DirectoryEntry = extern struct {
+    Name: [11]u8 align(1),
+    Attributes: u8 align(1),
+    _Reserved: u8 align(1),
+    CreatedTimeTenths: u8 align(1),
+    CreatedTime: u16 align(1),
+    CreatedDate: u16 align(1),
+    AccessedDate: u16 align(1),
+    FirstClusterHigh: u16 align(1),
+    ModifiedTime: u16 align(1),
+    ModifiedDate: u16 align(1),
+    FirstClusterLow: u16 align(1),
+    Size: u32 align(1),
+};
+
+var g_RootDirectory: []DirectoryEntry = undefined;
+var g_RootDirectoryEnd: u32 = undefined;
+
+fn readStruct(reader: *std.fs.File.Reader, T: type) !T {
+    var ret: [@sizeOf(T)]u8 = undefined;
+    _ = try reader.*.readStreaming(&ret);
+    return @bitCast(ret);
+}
+
+fn readBootSector(disk: *std.fs.File) !void {
+    var reader = disk.*.reader(&buffer);
+    g_BootSector = try readStruct(&reader, BootSector);
+}
+
+fn readSectors(disk: *std.fs.File, lba: u32, count: u32, bufferOut: *[]u8) !void {
+    const dest: []u8 = try std.heap.smp_allocator.alloc(u8, g_BootSector.BytesPerSector * count);
+    defer std.heap.smp_allocator.free(dest);
+    try disk.seekTo(lba * g_BootSector.BytesPerSector);
+    var reader = disk.readerStreaming(&buffer);
+    if (try reader.readStreaming(dest) != g_BootSector.BytesPerSector * count) return Errors.ReadSectors;
+    @memcpy(bufferOut.*, dest);
+}
+
+fn readFat(disk: *std.fs.File) !void {
+    g_Fat = try std.heap.smp_allocator.alloc(u8, g_BootSector.SectorsPerFat * g_BootSector.BytesPerSector);
+    errdefer std.heap.smp_allocator.free(g_Fat);
+    try readSectors(disk, g_BootSector.ReservedSectors, g_BootSector.SectorsPerFat, &g_Fat);
+}
+
+fn readRootDirectory(disk: *std.fs.File) !void {
+    const lba: u32 = g_BootSector.ReservedSectors + g_BootSector.SectorsPerFat * g_BootSector.FatCount;
+    const size: u32 = @sizeOf(DirectoryEntry) * g_BootSector.DirEntryCount;
+    var sectors: u32 = (size / g_BootSector.BytesPerSector);
+    if (@rem(size, g_BootSector.BytesPerSector) > 0)
+        sectors += 1;
+
+    g_RootDirectory = try std.heap.smp_allocator.alloc(DirectoryEntry, sectors * g_BootSector.BytesPerSector);
+    g_RootDirectoryEnd = lba + sectors;
+    try readSectors(disk, lba, sectors, @ptrCast(&g_RootDirectory));
+    for (g_RootDirectory) |Entry| {
+        std.log.debug("Entry: {any}, Entry.Name: {s}", .{
+            Entry, Entry.Name,
+        });
+    }
+}
+
+fn findFile(name: []const u8) !*DirectoryEntry {
+    for (g_RootDirectory, 0..) |Entry, i| {
+        std.log.debug("looking at {s}", .{Entry.Name});
+        if (std.mem.eql(u8, &Entry.Name, name))
+            return &g_RootDirectory[i];
+    }
+    return Errors.FileNotFound;
+}
+
+fn readFile(file: *DirectoryEntry, disk: *std.fs.File, outputBuffer: *[]u8) !void {
+    var currentCluster: u16 = file.FirstClusterLow;
+    const offset = g_BootSector.SectorsPerCluster * g_BootSector.BytesPerSector;
+    var i: usize = 0;
+
+    while (currentCluster < 0x0FF8) {
+        std.log.debug("currentCluster: {}", .{currentCluster});
+        const lba: u32 = g_RootDirectoryEnd + (currentCluster -% 2) * g_BootSector.SectorsPerCluster;
+        var sizedOutput = outputBuffer.*[i .. i + offset];
+        try readSectors(disk, lba, g_BootSector.SectorsPerCluster, &sizedOutput);
+        std.log.debug("sizedOutput: {s}", .{sizedOutput});
+        i += offset;
+
+        const fatindex = currentCluster * 3 / 2;
+
+        std.log.debug("@as(u16, g_Fat[fatindex]): {}, g_Fat[fatindex]: {}", .{ @as(*align(1) u16, @ptrCast(&g_Fat[fatindex])).*, g_Fat[fatindex] });
+        if (@rem(currentCluster, 2) == 0) {
+            currentCluster = @as(*align(1) u16, @ptrCast(&g_Fat[fatindex])).* & 0x0FFF;
+        } else {
+            currentCluster = @as(*align(1) u16, @ptrCast(&g_Fat[fatindex])).* >> 4;
+        }
+    }
+}
+
+pub fn main() !void {
+    var args = try std.process.argsWithAllocator(std.heap.smp_allocator);
+    defer args.deinit();
+    if (args.inner.count < 3) {
+        std.log.err("syntax: {s} <disk image> <file name>", .{std.os.argv[0]});
+        return Errors.InvalidSyntax;
+    }
+
+    _ = args.skip();
+
+    var disk = try std.fs.cwd().openFileZ(args.next().?, .{ .mode = .read_only });
+    std.log.debug("opened disk {any}", .{disk});
+
+    try readBootSector(&disk);
+    std.log.debug("read BootSector from disk, {any}", .{g_BootSector});
+
+    try readFat(&disk);
+    defer std.heap.smp_allocator.free(g_Fat);
+    std.log.debug("read file allocation table from disk, {any}", .{g_Fat});
+
+    try readRootDirectory(&disk);
+    defer std.heap.smp_allocator.free(g_RootDirectory);
+    std.log.debug("read root directory from disk, {any}", .{g_RootDirectory});
+
+    const fileEntry = try findFile(try std.fmt.allocPrint(std.heap.smp_allocator, "{s}", .{args.next().?}));
+
+    var fileBuffer = try std.heap.smp_allocator.alloc(u8, fileEntry.Size + g_BootSector.BytesPerSector);
+    @memset(fileBuffer, 0);
+    defer std.heap.smp_allocator.free(fileBuffer);
+
+    try readFile(fileEntry, &disk, &fileBuffer);
+    const stdout = std.fs.File.stdout();
+    defer stdout.close();
+    var writer = stdout.writer(&buffer);
+    try writer.interface.print("{s}\n", .{@as(*[:0]u8, @ptrCast(&fileBuffer)).*});
+
+    const newline = "\n";
+    _ = try std.posix.write(0, fileBuffer);
+    _ = try std.posix.write(0, newline);
+}

src/reference_implementations/test.txt

@@ -0,0 +1,3 @@
+testing file
+aabaacaadaaeaafaagaahaaiaajaakaalaamaanaaoaapaaqaaraasaataauaavaawaaxaayaaz
+tstngfl