;; otb.asm Octal To Binary
;; version 0.0.3a
;;
;; Copyright 2016 Kevin G. Austin
;;
;; Licensed under the Apache License, Version 2.0 (the "License");
;; you may not use this file except in compliance with the License.
;; You may obtain a copy of the License at
;; 
;;     http://www.apache.org/licenses/LICENSE-2.0
;; 
;; Unless required by applicable law or agreed to in writing, software
;; distributed under the License is distributed on an "AS IS" BASIS,
;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;; See the License for the specific language governing permissions and
;; limitations under the License.
;;
;; to build :
;; For 32 bit Linux systems
;; nasm -f elf otb.asm && ld -s otb.o -o otb
;; For 64 bit Linux systems
;; nasm -f elf64 otb.asm && ld -s otb.o -o otb
;;
;; Uncomment the next line for 32 bit systems
;; BITS 32
;; Uncomment the next line for 64 bit systems
;; BITS 64


section .bss
		ibufr:	resb	1		; reserve 1 byte buffer
		obufr:	resb	1		; reserve 1 byte buffer

section .text

		global _start

_start:		

;; Loop once for each char of output.

charloop:


		call	readchar		; get the first two bits


		mov	esi, ibufr		; set esi to buffer pointer
		mov	al, [esi]		; move buffer char to al

		cmp	al, 10
		je	charloop		; found newline

		sub	al, 0x30		; convert from printable
		shl	al, 6
		mov	esi, obufr		; set esi to buffer pointer
		mov	[byte esi], byte al	; save al buffer

		call	readchar		; get the middle three bits

		mov	esi, ibufr		; set esi to buffer pointer
		mov	bl, [esi]		; move buffer char to bl
		sub	bl, 0x30		; convert from printable
		shl	bl, 3
		mov	esi, obufr		; set esi to buffer pointer
		mov	al, [esi]		; move buffer char to al
		or	al, bl			; add middle three bits
		mov	esi, obufr		; set esi to buffer pointer
		mov	[byte esi], byte al	; save al buffer

		call	readchar		; get the last three bits

		mov	esi, ibufr		; set esi to buffer pointer
		mov	bl, [esi]		; move buffer char to bl
		sub	bl, 0x30		; convert from printable
		mov	esi, obufr		; set esi to buffer pointer
		mov	al, [esi]		; move buffer char to al
		or	al, bl			; add last three bits
		mov	esi, obufr		; set esi to buffer pointer
		mov	[byte esi], byte al	; put result back in buffer

		call	writechar

		call	readchar		; get the space

		jmp	charloop

done:
		xor	eax, eax			; clear eax
		inc	eax				; set syscall to exit (= 1)
		int	0x80				; syscall

writechar:
		mov	eax, 4			; syscall 4 = write
		mov	bl, 1			; 1 = stdout
		mov	ecx, obufr		; pointer to char buffer
		mov	dl, 1			; write 1 char
		int	0x80			; syscall
		or	eax, eax		; test return
		jle	done			; <= 0 means failure
		ret

readchar:
		mov	eax, 3			; syscall 3 = read
		mov	bl, 0			; 0 = stdin
		mov	ecx, ibufr		; pointer to buffer
		mov	dl, 1			; read 1 char
		int	0x80			; syscall

		or	eax, eax		; test return
		jle	done			; <= 0 means done

		mov	esi, ibufr		; pointer to buffer
		mov	al, [esi]		; move buffer char to al

		cmp	eax, 10
		je	endtests		; found newline

		cmp	eax, 32
		je	endtests		; found space

		cmp	eax, 47
		jle	done			; found illegal char

		cmp	eax, 58
		jge	done			; found illegal char
endtests:
		ret