initial code

README.md

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+# foxdos

b

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+NAME=foxdos
+
+rm $NAME
+
+mkdir -p obj
+
+# TODO this only works in root directory. might be good to just use a proper build system
+nasm -I. -Isrc src/boot.s -o obj/boot
+nasm -I. -Isrc src/kernel.s -o obj/kernel
+
+cat obj/boot obj/kernel > $NAME

config.s

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+NUMSEG equ 0 ; TODO build system should calculate this value
+
+; might be good to move one of these into the high memory area
+K_ADDR    equ 0x500  ; kernel load address
+STACK_SEG equ 0x1050

src/boot.s

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+; mothdos boot sector. should load kernel into segment 0x50 at 0x00. max size
+; would be 64k but i doubt a dos needs more than that. stack is placed at
+; 0x10500. i would suggest changing this to load the kernel into extended
+; a20-gate memory so as not to overwrite the boot sector with larger kernels
+; but i will leave that for you to do. code is public domain as always. i
+; have not tested any of this so your mileage using it may vary
+
+%include "config.s"
+
+[org 0x7C00]
+[bits 16]
+
+mov ax, K_ADDR << 4
+push ax ; setup for ds
+mov es, ax
+mov gs, ax
+mov fs, ax
+
+xor bx, bx
+mov ds, bx
+mov word [0x21*4], 2 ; see kernel_entry
+mov word [0x21*4+2], ax
+pop ds
+
+mov ax, 0x200 | NUMSEG
+mov cl, 2
+xor dh, dh
+int 13h
+
+; 64kb stack
+mov ax, STACK_SEG
+mov ss, ax
+xor sp, sp
+mov ax, sp
+not sp
+
+; kernel time(?)
+push ds
+push ax
+retf
+
+; TODO we have lots of space for init code here
+
+times 510 - ($-$$) db 0
+dw 0xAA55

src/int21.s

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+int21:
+	push ds
+	push cs
+	pop ds
+	; your handler goes here
+	; you could maybe make a jump table that takes ah as input?
+	pop ds
+	iret
+
+retcode: dw 0
+verify: db 0
+
+get_retcode:; ah=4dh
+	mov ax, [retcode]
+	retf 
+
+get_version:; ah=30h
+	mov ax, 8 ; if it is not zero indexed this indicates windows ME
+	xor bx, bx ; update: what does that comment mean
+	mov cx, bx
+	retf
+
+getverify:; ah=54h
+	mov al, [verify]
+	retf
+
+setverify:; ah=2eh
+	mov [verify], al
+	retf
+
+getint:; ah=35h
+	push ds
+	push di
+	push ax
+	xor ah, ah
+	shl ax, 2
+	mov di, ax
+	xor ax, ax
+	mov ds, ax
+	pop ax
+	mov word bx, [di]
+	mov word es, [di+2]
+	pop di
+	pop ds
+	retf
+
+setint:; ah=25h
+	pusha
+	xor ah, ah
+	shl ax, 2
+	mov di, ax
+	xor ax, ax
+	push ds
+	mov ds, ax
+	mov word [di], dx
+	mov word [di+2], ds
+	pop ds
+	popa
+	retf
+
+rdcmos:; input in al. TODO handle bcd here to abstractt it from the kernel
+	cli
+	cmp al, 9 ; not rtc register, do not wait. if this function bugs out
+	jg .rd    ; on a century boundary thats not on me. stop using dos
+	xchg al, ah
+.wait:	mov al, 0xA  ; msb specifies if rtc update is in progress
+	out 0x70, al ; TODO https://wiki.osdev.org/CMOS#RTC_Update_In_Progress
+	in al, 0x71
+	shl al, 1
+	jc .wait
+	xchg al, ah
+.rd:	out 0x70, al
+	in al, 0x71
+	sti
+	ret
+
+wrcmos:; register in bh, value in bl
+	cli
+	push ax
+	mov al, bh
+	or al, 0x80 ; nmi always on
+	out 0x70, al
+	mov al, bl
+	out 0x71, al
+	pop ax
+	sti
+	ret
+
+gettime:; ah=2ch
+	push ax
+	xor dl, dl
+
+	mov al, dl ; seconds
+	call rdcmos
+	mov dh, al
+
+	mov al, 2h ; minutes
+	call rdcmos
+	mov cl, al
+
+	mov al, 4h ; hours
+	call rdcmos
+	mov ch, al
+
+	shl al, 1 ; adjust for 12 hour time
+	jnc .end
+	add ch, 12
+
+.end:	pop ax
+	retf
+
+settime:; ah=2dh
+	push bx
+	mov bl, ch
+	mov bh, 0x4
+	call wrcmos
+	mov bl, cl
+	shr bh, 1
+	call wrcmos
+	mov bl, dh
+	cmp dl, 50 ; rtc cannot hold hundredths of a second so we round
+	jng .f
+	inc bl
+.f:	xor bh, bh
+	call wrcmos
+	xor al, al ; it probably succeeded, its fine. TODO am i missing return codes anywhere else
+	pop bx
+	iret

src/kernel.s

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+%include "config.s"
+[org K_ADDR]
+kernel_entry: ; MUST be a short jump due to loader config
+	jmp $ ; TODO stub because os doesnt actually exist yet
+
+%include "int21.s"