Add new under construction system calls to the readme and change the one for loading files to accomodate them, move the disk parameters to the main namespace in anticipation of restructuring the disk operations with the addition of the new system calls, rewrite parts of system to remove essentially duplicate code, and rewrite the explanation of the file system in the readme.

Co-authored-by: Jonas 'Sortie' Termansen <sortie@maxsi.org>

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LICENSE.MD

@@ -1,7 +1,7 @@
 MIT License
 ===========
 
-Copyright (c) 2021 CrazyEttin
+Copyright (c) 2021 CrazyEttin and contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a
 copy of this software and associated documentation files (the

README.MD

@@ -1,9 +1,11 @@
 EttinOS
 =======
 
-EttinOS is a minimalist 16-bit DOS-like hobbyist operating system for
-the IBM PC and compatible computers. Its git repository can be found at
-https://ahti.space/git/crazyettin/EttinOS.
+EttinOS is a minimalist 16-bit hobbyist disk operating system for the
+IBM Personal Computer and compatible machines. Its git repository can be
+found at https://ahti.space/git/crazyettin/EttinOS and that of
+EttinOS-extra, a collection of programs for the system, at
+https://ahti.space/git/crazyettin/EttinOS-extra.
 
 System requirements
 -------------------
@@ -11,44 +13,72 @@ System requirements
   * An Intel 8086 or compatible CPU
   * BIOS, or UEFI in legacy mode
   * 64 KiB of RAM
-  * A floppy disk drive
+  * 1-4 floppy disk drives (hard disk drives are not supported)
 
 Building
 --------
 
 Build dependencies:
   * A Unix-like operating system
-  * bash
   * coreutils
   * 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
-use the argument -1440. If you want to use another floppy disk format
-you will have to adjust the bootloader disk description tables and
-install the system manually. Hard disk drives are not supported.
+floppy disk image named EttinOS.img and a source image of the same
+format named EttinOS-src.img. To get 1440 KiB 3.5" images instead use
+the argument -1440. If you want to use another format you will have to
+build the system manually.
 
-Input
+Usage
 -----
 
-The EttinOS input system is inspired by typewriters. Typing a character
+The 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.
 
+EttinOS assigns the drives letters from A to D and uses a version of the
+MS-DOS 3.0 FAT12 file system (in actual use since 2.1) without support
+for file attributes and thus disk labels and subdirectories. Drive
+letters and file names are case-insensitive and the latter follow the
+8.3 format. Text files use CRLF line endings.
+
+Drives and files are specified as ([A-D]:) and ([A-D]:)FILENAME.EXT
+respectively. Specifying the current drive, indicated in the prompt, is
+optional.
+
+A command can be followed by arguments separated from eachother and the
+command with spaces. Extra spaces are ignored. Commands other than
+changing the current drive are stored as external programs: the command
+for a program is its file specification without the extension.
+
+Commands included in EttinOS:
+  * [A-D]:: Change the current drive.
+  * ECHO:   Print a message. Syntax: ECHO Message to be printed
+  * HELLO:  Print "Hello world!".
+  * LIST:   Print a list of the files on a drive. Syntax: LIST DRIVE
+  * PRINT:  Print a text file. Syntax: PRINT FILE
+
+Both LIST and PRINT page their output if all of it does not fit on the
+screen at the same time. Press any key other than escape to continue
+printing after each screen or escape to quit the 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.
+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 0x3000, the
+number of the current drive is loaded in DL, 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.
-  * Interrupt 0x21: Input and output:
+  * Interrupt 0x21: String operations:
       * AH = 0x0: Print a string ending in a null from SI.
       * AH = 0x1: Read a string ending in a null of at most AL
                   characters to DI until a return.
@@ -56,8 +86,37 @@ System calls:
                   CRLF.
       * AH = 0x3: Read a string ending in a null of at most AL
                   characters to DI until a return and print a CRLF.
+      * AH = 0x4: (Under construction) Convert a decimal string ending
+                  in a null at SI to a value in AL.
+      * AH = 0x5: (Under construction) Convert a value in AL to a
+                  decimal string ending in a null at DI.
+      * AH = 0x6: (Under construction) Convert a hexadecimal string
+                  ending in a null at SI to a value in AL.
+      * AH = 0x7: (Under construction) Convert a value in AL to a
+                  hexadecimal string ending in a null at DI.
   * Interrupt 0x22: Disk operations:
-      * AH = 0x0: Load a file named in SI as a string ending in a null
-                  to the offset BX and set AL to 0x0 if the load was
-                  succesfull and 0x1 if there was an error.
-      * AH = 0x1: Save a file (under construction).
+      * AH = 0x0: (Under construction) Load the directory of a drive
+                  named at SI as a string ending in a null to the offset
+                  BX and store the error codes in AL:
+                    * AL = 0x0: Succesful load
+                    * AL = 0x1: Drive not found
+                    * AL = 0x2: Unable to read disk
+      * AH = 0x1: (Under construction) Store a directory at the offset
+                  BX to a drive named at SI as a string ending in a
+                  null.
+      * AH = 0x2: Load a file named at SI as a string ending in a null
+                  to the offset BX and store the file size in CX and the
+                  error codes in AL:
+                    * AL = 0x0: Succesful load
+                    * AL = 0x1: Drive not found
+                    * AL = 0x2: Unable to read disk
+                    * AL = 0x4: File or command not found
+                    * AL = 0x8: Not enough memory
+      * AH = 0x3: (Under construction) Save a file.
+
+Known bugs
+----------
+
+  * Trying to access a disk of different format than the current one can
+    crash the system.
+  * Files beyond the ~40th one on the disk might not load properly.

make.sh

@@ -1,10 +1,8 @@
 #!/bin/sh
 
 rm -f EttinOS.img
-if [ ! -d "bin" ]
-then
-    mkdir bin
-fi
+rm -f EttinOS-src.img
+mkdir -p bin
 
 cd src/
 if [ "$1" = "-1440" ]
@@ -12,15 +10,42 @@ 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
+nasm LIST.ASM -f bin -o ../bin/LIST.BIN
+nasm PRINT.ASM -f bin -o ../bin/PRINT.BIN
 
 cd ..
 if [ "$1" = "-1440" ]
-    then mkfs.fat -C EttinOS.img 1440
-    else mkfs.fat -C EttinOS.img 360
+    then
+        mkfs.fat -C EttinOS.img 1440 1>/dev/null
+        mkfs.fat -C EttinOS-src.img 1440 1>/dev/null
+    else
+        mkfs.fat -C EttinOS.img 360 1>/dev/null
+        mkfs.fat -C EttinOS-src.img 360 1>/dev/null
+fi
+
+if which rw > /dev/null 2>&1
+    then rw -i bin/BOOT.BIN -o EttinOS.img -b 512 -c 1x
+    else dd if=bin/BOOT.BIN of=EttinOS.img conv=notrunc bs=512 count=1 1>/dev/null
 fi
-rw -i bin/BOOT.BIN -o EttinOS.img -b 512 -c 1x
 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 ::
+mcopy -i EttinOS.img bin/LIST.BIN ::
+mcopy -i EttinOS.img bin/PRINT.BIN ::
+
+mcopy -i EttinOS-src.img src/BOOT.ASM ::
+mcopy -i EttinOS-src.img src/SYSTEM.ASM ::
+mcopy -i EttinOS-src.img src/PRINTSTR.INC ::
+mcopy -i EttinOS-src.img src/READSTR.INC ::
+mcopy -i EttinOS-src.img src/PRINTLN.INC ::
+mcopy -i EttinOS-src.img src/READLN.INC ::
+mcopy -i EttinOS-src.img README.MD ::
+mcopy -i EttinOS-src.img LICENSE.MD ::
+mcopy -i EttinOS-src.img src/ECHO.ASM ::
+mcopy -i EttinOS-src.img src/HELLO.ASM ::
+mcopy -i EttinOS-src.img src/LIST.ASM ::
+mcopy -i EttinOS-src.img src/PRINT.ASM ::

src/BOOT.ASM

@@ -1,120 +1,136 @@
-CPU 8086
-ORG 0x7c00
+cpu 8086
+org 0x7c00
 
+;Jump to the code
 jmp start
+
+;Padding
 nop
 
-;Disk description tables
+;OEM label
+db "ETTINOS "
 
 %ifdef F1440
-;1.44 MB 3.5" floppy disk (enable with the argument -d F1440 when building)
-oemlabel        db "ETTINOS "
+
+;1.44 MB 3.5" floppy disk BPB (enable with the argument -d F1440)
 sectorsize      dw 0x200 ;bytes
 clustersize     db 0x1 ;sectors
-bootsectors     dw 0x1
+reservedsectors dw 0x1
 fats            db 0x2
 rootentries     dw 0xe0
 logicalsectors  dw 0xb40
 mediadescriptor db 0xf0
 sectorsperfat   dw 0x9
 sectorspertrack dw 0x12
-sides           dw 0x2
-hiddensectors   dd 0x0
-largesectors    dd 0x0
-driveid         dw 0x0
-drivesignature  db 0x29
-volumeid        dd 0x0
-volumelabel     db "ETTINOS    "
-filesystem      db "FAT12   "
+heads           dw 0x2
+hiddensectors   dw 0x0
 
 %else
-;360 KiB 5.25" floppy disk (default)
-oemlabel        db "ETTINOS "
+
+;360 KiB 5.25" floppy disk BPB (default)
 sectorsize      dw 0x200 ;bytes
 clustersize     db 0x2 ;sectors
-bootsectors     dw 0x1
+reservedsectors dw 0x1
 fats            db 0x2
 rootentries     dw 0x70
 logicalsectors  dw 0x2d0
 mediadescriptor db 0xfd
 sectorsperfat   dw 0x2
 sectorspertrack dw 0x9
-sides           dw 0x2
-hiddensectors   dd 0x0
-largesectors    dd 0x0
-driveid         dw 0x0
-drivesignature  db 0x29
-volumeid        dd 0x0
-volumelabel     db "ETTINOS    "
-filesystem      db "FAT12   "
+heads           dw 0x2
+hiddensectors   dw 0x0
 
 %endif
 
-start:
-
 ;Setup
-;Set up the data, stack, and extra segments
+;Set the segments
+start:
 mov ax, 0x0
 mov ds, ax
 mov ss, ax
 mov es, ax
-;Set up the stack
+;Set the stack
 cli
 mov sp, 0x0
 sti
 ;Store the boot drive number
-mov [bootdrive], dl
+mov [drive], dl
 
-;Load the root
-;Set the source
+;Calculate and store variables not found in the BPB
+;Start of the root
 mov ah, 0x0
 mov al, [fats]
 mul word [sectorsperfat]
-add ax, [bootsectors]
-push ax
-call calcsource
-;Set the destination
-mov si, buffer
-mov bx, si
-;Set the size
-push dx
+add ax, [reservedsectors]
+mov [rootstart], ax
+;Size of the root in sectors
 mov ax, [rootentries]
 mov dx, 0x20
 mul dx
 mov dx, 0x0
 div word [sectorsize]
-pop dx
+mov [rootsectors], ax
+;Start of data
+add ax, [rootstart]
+mov [datastart], ax
+
+;Load the root
+;Set the source
+mov ax, [rootstart]
+;Set the destination
+mov si, buffer
+mov bx, si
+;Set the size
+mov cx, [rootsectors]
+;Store the source and the loop counter in the stack
+loadrootsector:
 push ax
-;Load
+push cx
+;Set the source
+call calcsource
+;Set the size
+mov al, 0x1
+;Load a sector
 mov ah, 0x2
 int 0x13
+;Load the loop counter and the source from the stack
+pop cx
+pop ax
+;Set the next sector
+add ax, 0x1
+add bx, word [sectorsize]
+;Load the next sector
+loop loadrootsector
 
 ;Search the root for the system entry
 ;Set DI to the root
 mov di, buffer
-;Initialise the search loop
+;Set the number of root entries
 mov cx, word [rootentries]
+;Set the entry pointer
 mov ax, 0x0
+;Store the loop counter in the stack
 search:
-;Store CX in the stack
 push cx
 ;Check for the system entry
-mov si, sysfile
+mov si, file
 mov cx, 0xb
 rep cmpsb
-je loadentry
-;Set DI to the next entry
+je loadfat
+;Set DI at the next entry
 add ax, 0x20
 mov di, buffer
 add di, ax
-;Load CX from the stack
+;Load the loop counter from the stack
 pop cx
+;Search the next entry
 loop search
 
-;Print an error message if the system is not found
-mov si, errormsg
-printerror:
+;Print an error message and hang if the system is not found
+;Set SI at the file error message
+mov si, filerrormsg
 ;Load a character
+printstr:
 lodsb
 ;Check for the string end
 cmp al, 0x0
@@ -122,53 +138,61 @@ je $
 ;Print the character
 mov ah, 0xe
 int 0x10
-;Repeat
-jmp printerror
+;Print the next character
+jmp printstr
 
-;Load the system entry
-loadentry:
+;Load the FAT
 ;Load CX from the stack
+loadfat:
 pop cx
-;Store the first cluster
-mov ax, word [es:di+0xf]
-mov word [cluster], ax
+;Store the address of the first cluster
+mov ax, [di + 0xf]
+mov [cluster], ax
 ;Set the source
-mov ax, 0x1
+mov ax, [reservedsectors]
 call calcsource
 ;Set the destination
-mov di, buffer
-mov bx, di
+mov bx, buffer
 ;Set the size
 mov ax, [sectorsperfat]
-;Load
+;Load the FAT
 mov ah, 0x2
 int 0x13
 
-;Load the system file
-
 ;Load a cluster
 loadcluster:
 ;Set the source
-pop cx
-pop bx
 mov ax, word [cluster]
 sub ax, 0x2
 mul byte [clustersize]
-add ax, bx
-add ax, cx
-push bx
-push cx
-call calcsource
+add ax, [datastart]
 ;Set the destination
 mov bx, word [pointer]
 ;Set the size
-;mov al, 0x1
-mov al, [clustersize]
-;Load
+mov ch, 0x0
+mov cl, byte [clustersize]
+;Store the loop counter in the stack
+loadclustersector:
+push cx
+;Set the source
+call calcsource
+;Set the size
+push ax
+mov al, 0x1
+;Load a sector
 mov ah, 0x2
 int 0x13
+pop ax
+;Set the next sector
+add ax, 0x1
+add bx, [sectorsize]
+;Load the loop counter from the stack
+pop cx
+;Load the next sector
+loop loadclustersector
 
 ;Calculate the next cluster
+;Check if the cluster is even or odd
 mov ax, [cluster]
 mov dx, 0x0
 mov bx, 0x3
@@ -177,65 +201,87 @@ mov bx, 0x2
 div bx
 mov si, buffer
 add si, ax
-mov ax, word [ds:si]
+mov ax, word [si]
 or dx, dx
 jz even
-odd:
-shr ax, 1
-shr ax, 1
-shr ax, 1
-shr ax, 1
+;If the cluster is odd shift out the first four bits
+times 0x4 shr ax, 0x1
 jmp contcalc
+;If the cluster is even mask out the final four bits
 even:
 and ax, 0xfff
 contcalc:
-mov word [cluster], ax
+;Check for the file end
 cmp ax, 0xff8
 jge boot
+;Store the address of the next cluster
+mov word [cluster], ax
+;Set the destination of the next cluster
 mov ax, [sectorsize]
 mul word [clustersize]
 add word [pointer], ax
+;Load the next cluster
 jmp loadcluster
 
-;Clear the stack and boot the system
+;Boot the system
+;Pass the boot drive number to the system
 boot:
-;Clear
-pop cx
-pop bx
-;Boot
+mov dl, byte [drive]
+;Boot the system
 jmp 0x0:0x500
 
 ;Data
-bootdrive  db 0x0
-sysfile  db "SYSTEM  BIN"
-errormsg db "System not found", 0xd, 0xa, 0x0
-cluster  dw 0x0
-pointer  dw 0x500
+drive       db 0x0
+rootstart   dw 0x0
+rootsectors dw 0x0
+datastart   dw 0x0
+file        db "SYSTEM  BIN"
+filerrormsg db "System not found", 0xd, 0xa, 0x0
+cluster     dw 0x0
+pointer     dw 0x500
+
+;***
 
 ;Calculate the source arguments for loading data from the disk
 calcsource:
+
+;Store AX and BX in the stack
 push ax
 push bx
+
+;Calculate the cylinder, head, and sector
+;Store the logical sector in BX
 mov bx, ax
+;Calculate the sector
 mov dx, 0x0
 div word [sectorspertrack]
 add dl, 0x1
 mov cl, dl
+;Load the logical sector from BX
 mov ax, bx
+;Calculate the head and cylinder
 mov dx, 0x0
 div word [sectorspertrack]
 mov dx, 0x0
-div word [sides]
-mov dh, dl
-mov ch, al
+div word [heads]
+mov dh, dl ;Head
+mov ch, al ;Cylinder
+
+;Load the boot drive number
+mov dl, byte [drive]
+
+;Load BX and AX from the stack
 pop bx
 pop ax
-mov dl, byte [bootdrive]
+
+;Return
 ret
 
-;Pad the binary to a full sector and make the disk bootable
+;***
+
 ;Padding
 times 0x1fe-($-$$) db 0x0
+
 ;Boot signature
 dw 0xaa55
 

src/ECHO.ASM

@@ -0,0 +1,14 @@
+cpu 8086
+org 0x3000
+
+;Check for an empty tail
+cmp byte [si], 0x0
+je done
+
+;Print the message
+mov ah, 0x2
+int 0x21
+
+;Return to the system
+done:
+int 0x20

src/HELLO.ASM

@@ -1,11 +1,13 @@
-CPU 8086
-ORG 0x2000
+cpu 8086
+org 0x3000
 
-;Prints a hello world.
-mov si, .hello
+;Print a hello world.
+mov si, hello
 mov ah, 0x2
 int 0x21
+
+;Return to the system
 int 0x20
 
 ;Data
-.hello db "Hello world!", 0x0
+hello db "Hello world!", 0x0

src/LIST.ASM

@@ -0,0 +1,264 @@
+cpu 8086
+org 0x3000
+
+;Store the drive number
+mov [drive], dl
+
+;Check for an empty tail
+cmp byte [si], 0x0
+je loadvalues
+
+;Change the drive if needed
+;Check for a drive specification
+cmp byte [si + 0x1], ":"
+jne driverror
+cmp byte [si + 0x2], 0x0
+je changedrive
+cmp byte [si + 0x2], 0x20
+je changedrive
+;Print an error message
+jmp driverror
+changedrive:
+;Get the BIOS equipment list
+int 0x11
+;Get the number of floppy drives
+times 0x6 shr ax, 0x1
+and ax, 0x3
+inc ax
+;Set the loop and drive letter counters and the drive letter and number
+mov cx, ax
+mov di, driveletters
+mov al, [si]
+mov dl, 0x0
+;Check which drive to change to
+checkdrive:
+cmp al, [di]
+je contchdrive
+inc di
+cmp al, [di]
+je contchdrive
+inc di
+inc dl
+loop checkdrive
+;Print an error message
+jmp driverror
+;Change the drive
+contchdrive:
+mov [drive], dl
+
+;Load the disk description table
+;Set the source
+loadvalues:
+mov dl, [drive]
+mov ch, 0x0
+mov dh, 0x0
+mov cl, 0x1
+;Set the destination
+mov si, buffer
+mov bx, si
+;Set the size
+mov al, 0x1
+;Load the disk description table
+mov ah, 0x2
+int 0x13
+jc diskerror
+
+;Store the disk values used for the rest of the call
+mov ax, [buffer + 0xb]
+mov [sectorsize], ax
+mov ax, [buffer + 0xe]
+mov [reservedsectors], ax
+mov al, [buffer + 0x10]
+mov [fats], al
+mov ax, [buffer + 0x11]
+mov [rootentries], ax
+mov ax, [buffer + 0x16]
+mov [sectorsperfat], ax
+mov ax, [buffer + 0x18]
+mov [sectorspertrack], ax
+mov ax, [buffer + 0x1a]
+mov [heads], ax
+
+;Calculate and store variables not found in the BPB
+;Start of the root
+mov ah, 0x0
+mov al, [fats]
+mul word [sectorsperfat]
+add ax, [reservedsectors]
+mov [rootstart], ax
+;Size of the root in sectors
+mov ax, [rootentries]
+mov dx, 0x20
+mul dx
+mov dx, 0x0
+div word [sectorsize]
+mov [rootsectors], ax
+
+;Load the root
+;Set the source
+mov ax, [rootstart]
+;Set the destination
+mov si, buffer
+mov bx, si
+;Set the size
+mov cx, [rootsectors]
+;Store the source and the loop counter in the stack
+loadrootsector:
+push ax
+push cx
+;Set the source
+call calcsource
+;Set the size
+mov al, 0x1
+;Load a sector
+mov ah, 0x2
+int 0x13
+;Load the loop counter and the source from the stack
+pop cx
+pop ax
+;Set the next sector
+add ax, 0x1
+add bx, word [sectorsize]
+loop loadrootsector
+
+;List
+;Set SI to the root
+mov si, buffer
+;Initialise the loop
+mov cx, word [rootentries]
+mov ax, 0x0
+mov bl, 0x17
+;Store CX in the stack
+loop:
+push cx
+;Check for an empty entry
+cmp byte [si], 0xe5
+je skip
+;Check for the end of entries
+cmp byte [si], 0x0
+je done
+;store AX in the stack
+push ax
+;Print the main part of the name
+mov cx, 0x8
+push ax
+printname:
+mov al, [si]
+mov ah, 0xe
+int 0x10
+inc si
+loop printname
+;Print a space
+mov al, 0x20
+mov ah, 0xe
+int 0x10
+;Print the extension
+mov cx, 0x3
+printext:
+mov al, [si]
+mov ah, 0xe
+int 0x10
+inc si
+loop printext
+;Print a newline
+mov al, 0xd
+mov ah, 0xe
+int 0x10
+mov al, 0xa
+mov ah, 0xe
+int 0x10
+;Load AX from the stack
+pop ax
+;Check paging
+cmp bl, 0x0
+jz page
+dec bl
+;Set SI to the next entry
+skip:
+add ax, 0x20
+mov si, buffer
+add si, ax
+;Load CX from the stack
+pop cx
+loop loop
+page:
+push ax
+mov ah, 0x0
+int 0x16
+cmp al, 0x1b
+je done
+pop ax
+mov bl, 0x17
+jmp skip
+
+done:
+int 0x20
+
+driverror:
+mov si, driverrormsg
+mov ah, 0x2
+int 0x21
+jmp done
+
+diskerror:
+mov si, diskerrormsg
+mov ah, 0x2
+int 0x21
+jmp done
+
+;Data
+drive           db 0x0
+sectorsize      dw 0x0 ;bytes
+reservedsectors dw 0x0
+fats            db 0x0
+rootentries     dw 0x0
+sectorsperfat   dw 0x0
+sectorspertrack dw 0x0
+heads           dw 0x0
+rootstart       dw 0x0
+rootsectors     dw 0x0
+driveletters    db "AaBbCcDd"
+driverrormsg    db "Drive not found", 0x0
+diskerrormsg    db "Unable to read disk", 0x0
+
+;***
+
+;Calculate the source arguments for loading data from the disk
+calcsource:
+
+;Store AX and BX in the stack
+push ax
+push bx
+
+;Calculate the cylinder, head, and sector
+;Store the logical sector in BX
+mov bx, ax
+;Calculate the sector
+mov dx, 0x0
+div word [sectorspertrack]
+add dl, 0x1
+mov cl, dl
+;Load the logical sector from BX
+mov ax, bx
+;Calculate the head and cylinder
+mov dx, 0x0
+div word [sectorspertrack]
+mov dx, 0x0
+div word [heads]
+mov dh, dl ;Head
+mov ch, al ;Cylinder
+
+;Load the drive number
+mov dl, byte [drive]
+
+;Load BX and AX from the stack
+pop bx
+pop ax
+
+;Return
+ret
+
+;***
+
+;Buffer
+buffer:

src/LOADF.INC

@@ -1,48 +1,91 @@
-;Load a file named in SI as a string ending in a null to the offset BX and set AL to 0x0 if the load was succesful and 0x1 if there was an error.
+;Load a file named at SI as a string ending in a null to the offset BX
+;and store the file size in CX and the error codes in AL:
+;  * AL = 0x0: Succesful load
+;  * AL = 0x1: Drive not found
+;  * AL = 0x2: Unable to read disk
+;  * AL = 0x4: File or command not found
+;  * AL = 0x8: Not enough memory
 
 loadf:
 
-;Store the initial registers in the stack
-push ax
+;Store BX, DX, SI, and DI in the stack
 push bx
-push cx
 push dx
 push si
 push di
 
-;Store the offset
+;Store the current drive and offset
 mov word [.pointer], bx
+mov dl, byte [curdrive]
+mov byte [drive], dl
+
+;Change the drive if needed
+;Check for a drive specification
+cmp byte [si + 0x1], ":"
+jne .convert
+;Get the BIOS equipment list
+int 0x11
+;Get the number of floppy drives
+times 0x6 shr ax, 0x1
+and ax, 0x3
+inc ax
+;Set the loop and drive letter counters and the drive letter and number
+mov cx, ax
+mov di, driveletters
+mov al, [si]
+mov dl, 0x0
+;Check which drive to change to
+.checkdrive:
+cmp al, [di]
+je .contchdrive
+inc di
+cmp al, [di]
+je .contchdrive
+inc di
+inc dl
+loop .checkdrive
+;Set AL to 0x1, CX to 0x0, and print an error message
+mov si, .driverrormsg
+mov ah, 0x2
+int 0x21
+mov al, 0x1
+mov cx, 0x0
+jmp .done
+;Change the drive
+.contchdrive:
+mov [drive], dl
+;Move to the file name
+add si, 0x2
 
 ;Set DI at .file and initialise it with spaces
+.convert:
 mov di, .file
 mov cx, 0xb
 mov al, 0x20
 rep stosb
 sub di, 0xb
 
-;Convert .file into FAT formatting
-
-;Initialise the length counter for the main part of the name
+;Convert the main part of the file name into FAT format
+;Set the length counter
 mov bl, 0x8
-
-;Convert the main part of the file name
-.nameloop:
 ;Load a character
+.nameloop:
 lodsb
 ;Check for a period
 cmp al, 0x2e
 je .initext
 ;Check for everything else and convert to upper case
 call .checkconv
+;Load the next character
 jmp .nameloop
 
-;Convert the extension
+;Convert the extension into FAT format
+;Set DI and the length counter for the extension
 .initext:
-;Set DI and initialise the length counter for the extension
 mov bl, 0x3
 mov di, .file+0x8
-.extloop:
 ;Load a character
+.extloop:
 lodsb
 ;Check for a period
 push ax
@@ -51,119 +94,223 @@ je .error
 pop ax
 ;Check for everything else and convert to upper case
 call .checkconv
+;Load the next character
 jmp .extloop
-
-;Set the carry flag and print an error message if the file name is invalid
+;Set AL to 0x4, CX to 0x0, and print an error message
 .error:
 pop ax
-stc
-mov si, .errormsg
+mov si, .filerrormsg
 mov ah, 0x2
 int 0x21
+mov al, 0x4
+mov cx, 0x0
 jmp .done
 
 ;Find and load the file
-
 .load:
 pop ax
 
-;Load the root
+;Load the disk description table
 ;Set the source
-mov ah, 0x0
-mov al, [.fats]
-mul word [.sectorsperfat]
-add ax, [.bootsectors]
-push ax
-call .calcsource
+mov dl, [drive]
+mov ch, 0x0
+mov dh, 0x0
+mov cl, 0x1
 ;Set the destination
 mov si, buffer
 mov bx, si
 ;Set the size
-push dx
-mov ax, [.rootentries]
+mov al, 0x1
+;Load the disk description table
+mov ah, 0x2
+int 0x13
+jnc .storevalues
+;Set AL to 0x2, CX to 0x0, and print an error message
+mov si, .diskerrormsg
+mov ah, 0x2
+int 0x21
+mov al, 0x2
+mov cx, 0x0
+jmp .done
+
+;Store the disk values used for the rest of the call
+.storevalues:
+mov ax, [buffer + 0xb]
+mov [sectorsize], ax
+mov al, [buffer + 0xd]
+mov [clustersize], al
+mov ax, [buffer + 0xe]
+mov [reservedsectors], ax
+mov al, [buffer + 0x10]
+mov [fats], al
+mov ax, [buffer + 0x11]
+mov [rootentries], ax
+mov ax, [buffer + 0x16]
+mov [sectorsperfat], ax
+mov ax, [buffer + 0x18]
+mov [sectorspertrack], ax
+mov ax, [buffer + 0x1a]
+mov [heads], ax
+
+;Calculate and store variables not found in the BPB
+;Start of the root
+mov ah, 0x0
+mov al, [fats]
+mul word [sectorsperfat]
+add ax, [reservedsectors]
+mov [rootstart], ax
+;Size of the root in sectors
+mov ax, [rootentries]
 mov dx, 0x20
 mul dx
 mov dx, 0x0
-div word [.sectorsize]
-pop dx
+div word [sectorsize]
+mov [rootsectors], ax
+;Start of data
+add ax, [rootstart]
+mov [datastart], ax
+
+;Load the root
+;Set the source
+mov ax, [rootstart]
+;Set the destination
+mov si, buffer
+mov bx, si
+;Set the size
+mov cx, [rootsectors]
+;Store the source and the loop counter in the stack
+.loadrootsector:
 push ax
-;Load
+push cx
+;Set the source
+call .calcsource
+;Set the size
+mov al, 0x1
+;Load a sector
 mov ah, 0x2
 int 0x13
+;Load the loop counter and the source from the stack
+pop cx
+pop ax
+;Set the next sector
+add ax, 0x1
+add bx, word [sectorsize]
+;Load the next sector
+loop .loadrootsector
 
 ;Search the root for the file entry
 ;Set DI to the root
 mov di, buffer
-;Initialise the search loop
-mov cx, word [.rootentries]
+;Set the number of root entries
+mov cx, word [rootentries]
+;Set the entry pointer
 mov ax, 0x0
+;Store the loop counter in the stack
 .search:
-;Store CX in the stack
 push cx
 ;Check for the file entry
 mov si, .file
 mov cx, 0xb
 rep cmpsb
-je .loadentry
-;Set DI to the next entry
+je .checksize
+;Set DI at the next entry
 add ax, 0x20
 mov di, buffer
 add di, ax
-;Load CX from the stack
+;Load the loop counter from the stack
 pop cx
+;Search the next entry
 loop .search
-
-;Set the carry flag and print an error message if the file is not found
-stc
-mov si, .errormsg
+;Set AL to 0x4, CX to 0x0, and print an error message
+mov si, .filerrormsg
 mov ah, 0x2
 int 0x21
-jmp .clearstack
+mov al, 0x4
+mov cx, 0x0
+jmp .done
 
-;Load the file entry
-.loadentry:
+;Check and store the file size
 ;Load CX from the stack
+.checksize:
 pop cx
-;Store the first cluster
-mov ax, word [es:di+0xf]
-mov word [.cluster], ax
+;Check for files larger than 64 KiB
+cmp word [di + 0x13], 0x0
+jne .sizerror
+;Get the cluster size in bytes
+mov ah, 0x0
+mov al, [clustersize]
+mul word [sectorsize]
+mov bx, ax
+;Store the file size
+mov ax, [di + 0x11]
+mov word [.size], ax
+;Check for files smaller than 64 KiB but too large to fit into memory
+jc .sizerror
+dec ax
+mov dx, 0x0
+div bx
+mul bx
+add ax, [.pointer]
+jnc .loadfat
+;Set AL to 0x8, CX to 0x0, and print an error message
+.sizerror:
+mov si, .sizerrormsg
+mov ah, 0x2
+int 0x21
+mov al, 0x8
+mov cx, 0x0
+jmp .done
+
+;Load the FAT
+.loadfat:
+;Store the address of the first cluster
+mov ax, [di + 0xf]
+mov [.cluster], ax
 ;Set the source
-mov ax, 0x1
+mov ax, [reservedsectors]
 call .calcsource
 ;Set the destination
-mov di, buffer
-mov bx, di
+mov bx, buffer
 ;Set the size
-mov ax, [.sectorsperfat]
-;Load
+mov ax, [sectorsperfat]
+;Load the FAT
 mov ah, 0x2
 int 0x13
 
-;Load the file
-
 ;Load a cluster
 .loadcluster:
 ;Set the source
-pop cx
-pop bx
 mov ax, word [.cluster]
 sub ax, 0x2
-mul byte [.clustersize]
-add ax, bx
-add ax, cx
-push bx
-push cx
-call .calcsource
+mul byte [clustersize]
+add ax, [datastart]
 ;Set the destination
 mov bx, word [.pointer]
 ;Set the size
-;mov al, 0x1
-mov al, [.clustersize]
-;Load
+mov ch, 0x0
+mov cl, byte [clustersize]
+;Store the loop counter in the stack
+.loadsector:
+push cx
+;Set the source
+call .calcsource
+;Set the size
+push ax
+mov al, 0x1
+;Load a sector
 mov ah, 0x2
 int 0x13
+pop ax
+;Set the next sector
+add ax, 0x1
+add bx, word [sectorsize]
+;Load the loop counter from the stack
+pop cx
+;Load the next sector
+loop .loadsector
 
 ;Calculate the next cluster
+;Check if the cluster is even or odd
 mov ax, [.cluster]
 mov dx, 0x0
 mov bx, 0x3
@@ -172,71 +319,64 @@ mov bx, 0x2
 div bx
 mov si, buffer
 add si, ax
-mov ax, word [ds:si]
+mov ax, word [si]
 or dx, dx
 jz .even
-shr ax, 1
-shr ax, 1
-shr ax, 1
-shr ax, 1
+;If the cluster is odd shift out the first four bits
+times 0x4 shr ax, 0x1
 jmp .contcalc
+;If the cluster is even mask out the final four bits
 .even:
 and ax, 0xfff
 .contcalc:
-mov word [.cluster], ax
+;Check for the file end
 cmp ax, 0xff8
-jge .clearcarry
-mov ax, [.sectorsize]
-mul word [.clustersize]
+jge .success
+;Store the address of the next cluster
+mov word [.cluster], ax
+;Set the destination of the next cluster
+mov ax, [sectorsize]
+mul word [clustersize]
 add word [.pointer], ax
+;Load the next cluster
 jmp .loadcluster
 
-;Clear the carry flag if the load was succesful
-.clearcarry:
-clc
+;Set AL to 0x0 and load the file size to CX if the load was succesful
+.success:
+mov al, 0x0
+mov cx, [.size]
 
 ;Clear the stack
-.clearstack:
-pop cx
-pop bx
-
 .done:
 
-;Load the initial registers from the stack
+;Load DI, SI, DX, and BX from the stack
 pop di
 pop si
 pop dx
-pop cx
 pop bx
-pop ax
 
-;Set AL to 0x1 if there was an error and to 0x0 otherwise and return
-jc .setal
-mov al, 0x0
-iret
-.setal:
-mov al, 0x1
+;Set AH to its initial value
+mov ah, 0x0
+
+;Return
 iret
 
 ;Data
-.file     times 0xb db 0x20
-.errormsg db "File not found", 0x0
-.cluster  dw 0x0
-.pointer  dw 0x0
+.file            times 0xb db 0x20
+.size            dw 0x0
+.cluster         dw 0x0
+.pointer         dw 0x0
+.driverrormsg    db "Drive not found", 0x0
+.diskerrormsg    db "Unable to read disk", 0x0
+.filerrormsg     db "File or command not found", 0x0
+.sizerrormsg     db "Not enough memory", 0x0
 
-;These are temporary until i write something to load them from the disk itself
-.bootdrive       db 0x0
-.sectorsize      dw 0x200 ;bytes
-.clustersize     db 0x2 ;sectors
-.bootsectors     dw 0x1
-.fats            db 0x2
-.rootentries     dw 0x70
-.sectorsperfat   dw 0x2
-.sectorspertrack dw 0x9
-.sides           dw 0x2
+;***
 
 ;Check the file name and convert to upper case
 .checkconv:
+
+;Check for the string end, length limit, and invalid characters
 ;Check for the string end
 cmp al, 0x0
 je .load
@@ -268,6 +408,8 @@ jmp .error
 .contcheck3:
 cmp al, 0x7c
 je .error
+
+;Check for lower case and convert it to upper case
 ;Check for lower case
 cmp al, 0x61
 jl .storech
@@ -275,30 +417,50 @@ cmp al, 0x7a
 jg .storech
 ;Convert lower to upper case
 sub al, 0x20
-.storech:
+
 ;Store the character
+.storech:
 stosb
-;Increase the counter
+
+;Decrease the counter
 dec bl
+
+;Return
 ret
 
+;***
+
 ;Calculate the source arguments for loading data from the disk
 .calcsource:
+
+;Store AX and BX in the stack
 push ax
 push bx
+
+;Calculate the cylinder, head, and sector
+;Store the logical sector in BX
 mov bx, ax
+;Calculate the sector
 mov dx, 0x0
-div word [.sectorspertrack]
+div word [sectorspertrack]
 add dl, 0x1
 mov cl, dl
+;Load the logical sector from BX
 mov ax, bx
+;Calculate the head and cylinder
 mov dx, 0x0
-div word [.sectorspertrack]
+div word [sectorspertrack]
 mov dx, 0x0
-div word [.sides]
-mov dh, dl
-mov ch, al
+div word [heads]
+mov dh, dl ;Head
+mov ch, al ;Cylinder
+
+;Load the drive number
+mov dl, byte [drive]
+
+;Load BX and AX from the stack
 pop bx
 pop ax
-mov dl, byte [.bootdrive]
+
+;Return
 ret

src/PRINT.ASM

@@ -0,0 +1,101 @@
+cpu 8086
+org 0x3000
+
+;Load a file and print it
+
+;Set the stack
+cli
+mov sp, stack + 0x100
+sti
+
+;Check for an empty tail
+;Check
+cmp byte [si], 0x0
+jne extract
+;Print an error message and abort if the tail is empty
+mov si, errormsg
+mov ah, 0x2
+int 0x21
+je done
+
+;Find the end of the filename and add a null if needed
+;Set DI at the tail
+extract:
+mov di, si
+findend:
+;Check for the string end
+cmp byte [di], 0x0
+je load
+;Check for a space
+cmp byte [di], 0x20
+je addnull
+inc di
+jmp findend
+;Add a null
+addnull:
+mov al, 0x0
+stosb
+
+;Load the file
+;Load
+load:
+mov bx, stack + 0x100
+mov ah, 0x2
+int 0x22
+;Check for errors
+cmp al, 0x0
+jne done
+
+;Print the file
+;Setup
+mov bh, 0x0
+mov bl, 0x18
+mov dl, 0x50
+mov si, stack + 0x100
+;Decrease the character counter
+print:
+dec dl
+;Load the current character
+lodsb
+;Print the character
+mov ah, 0xe
+int 0x10
+;Check for a hard line end
+cmp al, 0xa
+je linecount
+;Check for a soft line end
+cmp dl, 0x0
+je linecount
+;Check paging
+contprint:
+cmp bl, 0x0
+jz page
+;Repeat for the next character
+loop print
+jmp done
+;Decrease the line counter and reset the character one
+linecount:
+dec bl
+mov dl, 0x50
+jmp contprint
+;Page
+page:
+push ax
+mov ah, 0x0
+int 0x16
+cmp al, 0x1b
+je done
+pop ax
+mov bl, 0x18
+mov dl, 0x50
+loop print
+
+;Return to the system
+done:
+int 0x20
+
+;Data
+errormsg db "File not found", 0x0
+
+;Stack
+stack:

src/PRINTLN.INC

@@ -1,5 +1,4 @@
 ;Print a string ending in a null from SI followed by a CRLF.
-
 println:
 
 ;Print the string
@@ -9,4 +8,5 @@ int 0x21
 ;Print a CRLF
 call printcrlf
 
+;Return
 iret

src/PRINTSTR.INC

@@ -1,14 +1,13 @@
 ;Print a string ending in a null from SI.
-
 printstr:
 
-;Store the initial registers in the stack
+;Store AX and SI in the stack
 push ax
 push si
 
 ;Print the string
-.loop:
 ;Load the current character
+.loop:
 lodsb
 ;Check for the string end
 cmp al, 0x0
@@ -16,13 +15,13 @@ je .done
 ;Print the character
 mov ah, 0xe
 int 0x10
-;Repeat for the next character
+;Print the next character
 jmp .loop
 
-.done:
-
 ;Load the initial registers from the stack
+.done:
 pop si
 pop ax
 
+;Return
 iret

src/READLN.INC

@@ -1,5 +1,5 @@
-;Read a string ending in null of at most AL characters to DI until a return and print a CRLF.
-
+;Read a string ending in a null of at most AL characters to DI until a
+;return and print a CRLF.
 readln:
 
 ;Read the string
@@ -9,4 +9,5 @@ int 0x21
 ;Print a CRLF
 call printcrlf
 
+;Return
 iret

src/READSTR.INC

@@ -1,8 +1,8 @@
-;Read a string ending in a null of at most AL characters to DI until a return.
-
+;Read a string ending in a null of at most AL characters to DI until a
+;return.
 readstr:
 
-;Store the initial registers in the stack
+;Store AX, BX, CX, DX, and DI in the stack
 push ax
 push bx
 push cx
@@ -20,12 +20,16 @@ rep stosb
 pop ax
 push ax
 sub di, ax
-;Initialise the cursor pointer in BL and clear BH
+;Store the last column number
+mov ah, 0xf
+int 0x10
+dec ah
+mov [.lastcolumn], ah
+;Set the cursor pointer in BL and clear BH
 mov bx, 0x1
 
-.loop:
-
 ;Read a keypress
+.loop:
 mov ah, 0x0
 int 0x16
 
@@ -55,9 +59,9 @@ je .loop
 
 ;Store and print a character
 .character:
-;Store the  character
+;Store
 stosb
-;Print the character
+;Print
 mov ah, 0xe
 int 0x10
 ;Move the cursor pointer
@@ -87,23 +91,21 @@ dec di
 dec bl
 jmp .loop
 
-;Finish reading the string
-.return:
-
-;Find and remove trailing spaces
+;Check for and remove trailing spaces
 ;Go to the end of the input
+.return:
 pop ax
 mov bh, 0x0
 sub ax, bx
 push di
 add di, ax
+;Check for trailing spaces
 .findtrailing:
-;Check for a trailing space
 cmp byte [di], 0x20
 je .deltrailing
 jmp .end
+;Remove trailing spaces
 .deltrailing:
-;Delete a trailing space
 mov al, 0x0
 stosb
 sub di, 0x2
@@ -122,83 +124,102 @@ inc di
 inc bl
 jmp .findend
 
+;Load DI, DX, CX, BX, and AX from the stack
 .done:
-
-;Load the initial registers from the stack
 pop di
 pop dx
 pop cx
 pop bx
 pop ax
 
+;Return
 iret
 
-;Move the cursor forward
+;Data
+.lastcolumn db 0x0
 
-;Move forward within a line
+;***
+
+;Move the cursor forward
 .nextchar:
+
 ;Get the cursor position
 mov ah, 0x3
 int 0x10
+
 ;Check for the end of the line
-cmp dl, 0x4f
+cmp dl, [.lastcolumn]
 je .nextln
-;Move
+
+;Move the cursor forward within a line
 inc dl
 mov ah, 0x2
 int 0x10
+
+;Return
 ret
 
-;Move to the beginning of the next line
-.nextln:
+;Move the cursor to the beginning of the next line
 ;Check if the current line is the last on screen
+.nextln:
 cmp dh, 0x18
 je .scroll
-;Move
+;Move the cursor
 mov ah, 0x2
 inc dh
 mov dl, 0x0
 int 0x10
+
+;Return
 ret
 
 ;Scroll the screen up by one line
 .scroll:
-;Scroll
 mov ah, 0x6
 mov al, 0x1
 mov bh, 0x7
 mov ch, 0x0
 mov cl, 0x0
 mov dh, 0x18
-mov dl, 0x4f
+mov dl, [.lastcolumn]
 int 0x10
+
 ;Move to the beginning of the new line
 mov ah, 0x2
 mov bh, 0x0
 mov dl, 0x0
 int 0x10
+
+;Return
 ret
 
-;Move the cursor backward
+;***
 
-;Move backward within a line
+;Move the cursor backward
 .prevchar:
+
 ;Get the cursor position
 mov ah, 0x3
 int 0x10
+
 ;Check for the beginning of the line
 cmp dl, 0x0
 je .prevln
-;Move
+
+;Move the cursor backward within a line
 dec dl
 mov ah, 0x2
 int 0x10
+
+;Return
 ret
 
-;Move to the end of the previous line
+;Move the cursor to the end of the previous line
 .prevln:
 mov ah, 0x2
 dec dh
-mov dl, 0x4f
+mov dl, [.lastcolumn]
 int 0x10
+
+;Return
 ret

src/SYSTEM.ASM

@@ -1,6 +1,7 @@
-CPU 8086
-ORG 0x500
+cpu 8086
+org 0x500
 
+;Jump to the code
 jmp start
 
 ;Interrupt handler
@@ -20,22 +21,21 @@ je readln
 iret
 ;Disk operations
 int0x22:
-cmp ah, 0x0
+cmp ah, 0x2
 je loadf
 ;To do: savef
 iret
 
-;System calls
+;System call includes
 %include "PRINTSTR.INC"
 %include "READSTR.INC"
 %include "PRINTLN.INC"
 %include "READLN.INC"
 %include "LOADF.INC"
 
-start:
-
 ;Set up the interrupt vectors
 ;Interrupt 0x20 offset
+start:
 mov ax, int0x20
 mov [0x80], ax
 ;Interrupt 0x21 offset
@@ -50,6 +50,11 @@ mov [0x82], ax
 mov [0x86], ax
 mov [0x8a], ax
 
+;Store the boot drive number as the current drive and set the current
+;drive letter
+mov [curdrive], dl
+call setcurdriveletter
+
 ;Print a welcome message
 mov si, welcomemsg
 mov ah, 0x2
@@ -63,62 +68,226 @@ mov sp, 0x0
 sti
 
 ;Prompt for and read a command
+;Print the current drive letter
+mov si, curdriveletter
+mov ah, 0x0
+int 0x21
 ;Print a prompt
 mov si, prompt
 mov ah, 0x0
 int 0x21
 ;Read
 mov di, input
-mov al, 0x4e
+mov al, 0x4c
 mov ah, 0x3
 int 0x21
 
-;Load an execute the program
 ;Check for an empty command
 cmp byte [input], 0x0
 jz shell
-;Load
-mov bx, 0x2000
+
+;Set SI at the command
+;Set SI at input
 mov si, input
-mov ah, 0x0
+;Ignore leading spaces
+call ignoreleading
+
+;Check for a current drive change command
+cmp byte [si + 0x1], ":"
+jne extract
+cmp byte [si + 0x2], 0x0
+je changecurdrive
+cmp byte [si + 0x2], 0x20
+je changecurdrive
+
+;Extract the specification of the program file
+extract:
+;Set DI at program
+mov di, program
+;Initialise program with spaces
+mov cx, 0xe
+mov al, 0x20
+rep stosb
+sub di, 0xe
+;Set the length counter
+mov bl, 0xa
+;Load a character
+specloop:
+lodsb
+;Check for the string end
+cmp al, 0x0
+je addext
+;Check for a space
+cmp al, 0x20
+je addext
+;Check for the length limit
+cmp bl, 0x0
+je cmderror
+;Store the character
+stosb
+;Decrease the counter
+dec bl
+;Extract the next character
+jmp specloop
+;Store the start of the command tail in the stack
+addext:
+push si
+;Add extension to the specification
+mov si, extension
+mov cx, 0x5
+rep movsb
+
+;Load and execute the program
+;Load the program
+mov bx, 0x3000
+mov si, program
+mov ah, 0x2
 int 0x22
 ;Check for errors
-cmp al, 0x1
-je error
-;Execute
-jmp 0x2000
+cmp al, 0x0
+jne shell
+;Pass the current drive and command tail to the program
+mov dl, [curdrive]
+pop si
+call ignoreleading
+;Execute the program
+jmp 0x3000
 
-;Print an error message and return to the shell
-error:
-mov bh, 0x0
-mov ah, 0x3
-int 0x10
-dec dh
-mov ah, 0x2
-int 0x10
-mov si, errormsg
+;Print a command error message and return to the shell
+cmderror:
+mov si, cmderrormsg
 mov ah, 0x2
 int 0x21
 jmp shell
 
-;Data
+;Change the current drive
+;Get the BIOS equipment list
+changecurdrive:
+int 0x11
+;Get the number of floppy drives
+times 0x6 shr ax, 0x1
+and ax, 0x3
+inc ax
+;Set the loop and drive letter counters and the drive letter and number
+mov cx, ax
+mov di, driveletters
+mov al, [si]
+mov dl, 0x0
+;Check which drive to change to
+checkdrive:
+cmp al, [di]
+je contchdrive
+inc di
+cmp al, [di]
+je contchdrive
+inc di
+inc dl
+loop checkdrive
+;Print a drive error message and return to the shell
+mov si, driverrormsg
+mov ah, 0x2
+int 0x21
+jmp shell
+;Change the drive
+contchdrive:
+mov [curdrive], dl
+call setcurdriveletter
+jmp shell
+
+;Welcome message
 welcomemsg db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
-prompt     db "> ", 0x0
-errormsg   db "Unknown command", 0x0
-input      times 0x4e db 0x0
-crlf       db 0xd, 0xa, 0x0
+
+;Drive stuff
+curdrive     db 0x0
+driveletters db "AaBbCcDd"
+
+;Shell
+curdriveletter db "?:", 0x0
+prompt         db "> ", 0x0
+input          times 0x4c db 0x0
+program        times 0xf db 0x0
+extension      db ".BIN", 0x0
+cmderrormsg    db "File or command not found", 0x0
+driverrormsg   db "Drive not found", 0x0
+
+;Disk parameters
+drive           db 0x0
+sectorsize      dw 0x0 ;bytes
+clustersize     db 0x0 ;sectors
+reservedsectors dw 0x0
+fats            db 0x0
+rootentries     dw 0x0
+sectorsperfat   dw 0x0
+sectorspertrack dw 0x0
+heads           dw 0x0
+rootstart       dw 0x0
+rootsectors     dw 0x0
+datastart       dw 0x0
+
+;***
+
+;Set the drive letter
+setcurdriveletter:
+
+;Set the drive number and letter counters
+mov dh, 0x0
+mov si, driveletters
+
+;Check the drive number
+.checkdrive:
+cmp dl, dh
+je .set
+add si, 0x2
+inc dh
+jmp .checkdrive
+
+;Set the drive letter
+.set:
+mov al, [si]
+mov byte [curdriveletter], al
+
+;Return
+ret
+
+;***
+
+;Ignore leading spaces in the command and its tail
+ignoreleading:
+
+;Check for a space
+lodsb
+cmp al, 0x20
+je ignoreleading
+
+;Set SI to the first non-space character
+dec si
+
+;Return
+ret
+
+;***
 
 ;Print a CRLF
 printcrlf:
-;Store the initial registers in the stack
+
+;Store SI in the stack
 push si
+
 ;Print the CRLF
-mov si, crlf
+mov si, .crlf
 mov ah, 0x0
 int 0x21
-;Load the initial registers from the stack
+
+;Load SI from the stack
 pop si
+
+;Return
 ret
 
+;Data
+.crlf db 0xd, 0xa, 0x0
+
+;***
+
 ;File system buffer
 buffer: