Write echo, update the readme with information on how the commands work, and replace a lodsb stosb loop in the shell with a movsb one.

README.MD

@@ -19,10 +19,10 @@ Building
 Build dependencies:
   * A Unix-like operating system
   * coreutils
-  * rw (optional)
   * dosfstools
   * mtools
   * nasm
+  * rw (optional)
 
 Running make.sh will build EttinOS and create a bootable 360 KiB 5.25"
 floppy disk image named EttinOS.img. To get a 1.44 MB 3.5" one instead
@@ -37,14 +37,32 @@ The EttinOS input system is inspired by typewriters. Typing a character
 overwrites the cursor location and the erase (=tab) key erases it. The
 space and backspace keys move the cursor.
 
+Commands
+--------
+
+EttinOS commands consist of the case-insensitive command itself followed
+by a tail separated with a space. Extra spaces, be they leading,
+trailing, or between a command and its tail, are ignored. What the tail
+contains and how it should be formatted depends on the command. Tails on
+commands that do not use them are ignored. All commands except for
+changing the drive are stored as external programs, and you must be on
+the same drive as the program file to execute one. A command for a
+program does not include its filename extension.
+
+Commands included in EttinOS by default:
+  * [A-D]:: Changes the drive.
+  * ECHO:   Echoes its tail.
+  * HELLO:  A hello world program.
+
 Programming
 -----------
 
 EttinOS has a flat address space of 64 KiB. The data, stack, and
 extra segments are set at the beginning of the RAM and the system stack
-at the end of the address space. Programs are loaded at address 0x2000.
-The stack is reset back to the end of the address space after a program
-has finished running.
+at the end of the address space. Programs are loaded at address 0x2000
+and SI is pointed at the command tail, a string ending in a null, when
+the program is executed. The stack is reset back to the end of the
+address space after a program has finished running.
 
 System calls:
   * Interrupt 0x20: Return to the shell.

make.sh

@@ -9,6 +9,7 @@ if [ "$1" = "-1440" ]
     else nasm BOOT.ASM -f bin -o ../bin/BOOT.BIN
 fi
 nasm SYSTEM.ASM -f bin -o ../bin/SYSTEM.BIN
+nasm ECHO.ASM -f bin -o ../bin/ECHO.BIN
 nasm HELLO.ASM -f bin -o ../bin/HELLO.BIN
 
 cd ..
@@ -22,6 +23,7 @@ if which rw > /dev/null 2>&1
     else dd if=bin/BOOT.BIN of=EttinOS.img conv=notrunc bs=512 count=1 1>/dev/null
 fi
 mcopy -i EttinOS.img bin/SYSTEM.BIN ::
-mcopy -i EttinOS.img bin/HELLO.BIN ::
 mcopy -i EttinOS.img README.MD ::
 mcopy -i EttinOS.img LICENSE.MD ::
+mcopy -i EttinOS.img bin/ECHO.BIN ::
+mcopy -i EttinOS.img bin/HELLO.BIN ::

src/ECHO.ASM

@@ -0,0 +1,12 @@
+CPU 8086
+ORG 0x2000
+
+;Echoes its command tail
+;Check for an empty tail
+cmp byte [si], 0x0
+je done
+;Echo
+mov ah, 0x2
+int 0x21
+done:
+int 0x20

src/LOADF.INC

@@ -22,10 +22,10 @@ sub di, 0xb
 
 ;Convert .file into FAT formatting
 
-;Initialise the length counter for the main part of the name
+;Initialise the length counter for the main part of the filename
 mov bl, 0x8
 
-;Convert the main part of the file name
+;Convert the main part of the filename
 .nameloop:
 ;Load a character
 lodsb
@@ -53,7 +53,7 @@ pop ax
 call .checkconv
 jmp .extloop
 
-;Set the carry flag if the file name is invalid
+;Set the carry flag if the filename is invalid
 .error:
 pop ax
 stc
@@ -260,7 +260,7 @@ iret
 .cluster         dw 0x0
 .pointer         dw 0x0
 
-;Check the file name and convert to upper case
+;Check the filename and convert to upper case
 .checkconv:
 ;Check for the string end
 cmp al, 0x0

src/SYSTEM.ASM

@@ -126,10 +126,7 @@ addext:
 push si
 mov si, extension
 mov cx, 0x5
-.loop:
-lodsb
-stosb
-loop .loop
+rep movsb
 
 ;Load and execute the program
 ;Load