Add DOS support

.gitignore

@@ -1,4 +1,5 @@
-bootmine
+*.img
+*.com
 bx_enh_dbg.ini
 lst
 TAGS

Makefile

@@ -3,19 +3,30 @@
 QEMU = qemu-system-i386
 BOCHS = bochs
 NASM = nasm
+DOSBOX = dosbox
+DOSEMU = dosemu
 
-.PHONY: all clean qemu bochs
+.PHONY: all clean qemu bochs dosbox dosemu
 
-all: bootmine
+all: bootmine.img bootmine.com
 
-bootmine: mine.asm
+bootmine.img: mine.asm
 	$(NASM) $< -o $@
 
+bootmine.com: mine.asm
+	$(NASM) -DDOS $< -o $@
+
 clean:
-	rm -f bootmine
+	rm -f bootmine.img bootmine.com
 
-qemu: bootmine
+qemu: bootmine.img
 	$(QEMU) $<
 
-bochs: bootmine
+bochs: bootmine.img
 	$(BOCHS) -q -f bochsrc.txt
+
+dosbox: bootmine.com
+	$(DOSBOX) $<
+
+dosemu: bootmine.com
+	$(DOSEMU) $<

README.md

@@ -25,7 +25,7 @@ make
 
 ## Installing
 
-This 512-byte file `bootmine`, can be written to the first sector of a floppy disk (or USB drive), with a command like `dd if=bootmine of=/dev/sdb`. Keep in mind that this will effectively destroy all data on the drive.
+This 512-byte file `bootmine.img`, can be written to the first sector of a floppy disk (or USB drive), with a command like `dd if=bootmine.img of=/dev/sdb`. Keep in mind that this will effectively destroy all data on the drive.
 
 ### Emulation
 

mine.asm

@@ -12,6 +12,9 @@ cpu 686
 ;; Words in 16 bit x86 are 2 bytes
 %assign WordSize 2
 
+;; Load address of DOS COM executables (relative to the cs segment)
+%assign DosComLoadAddress 0x100
+
 ;; This is the value to store in segment register to access the VGA text buffer.
 ;; In 16 bit x86, segmented memory accesses are of the form:
 ;;
@@ -68,7 +71,13 @@ cpu 686
 ;; halves the amount of bombs.
 %assign BombFreq 0b111
 
+;; BootMine is supported as both a DOS game and a boot sector game :)
+;; To build for DOS, run NASM with -DDOS and name the file with a .com extension
+%ifdef DOS
+org DosComLoadAddress
+%else
 org BootSector.Begin
+%endif
 
 ;; Entry point: set up graphics and run game
 BootMine:
@@ -167,8 +176,9 @@ PopulateTextBuf:
   ; Save di since it will be clobbered later
   push di
   ; Load adjacent cell offset from Dirs array into ax. Since the offset register
-  ; is bp, the segment register used is ss, which zero. This makes the memory
-  ; access relative to the start of RAM instead of the start of the text buffer.
+  ; is bp, the segment register used is ss, which is the same as cs. This makes
+  ; the memory access relative to BootMine's code instead of the start of the
+  ; text buffer.
   movsx ax, byte [bp + Dirs - 1]
   ; Set di = pointer to adjacent cell
   add di, ax
@@ -527,11 +537,17 @@ GameOver:
 ;;   * bp - pointer to string
 ;;   * bx - color of string
 GameEndHelper:
+  ; Reset es segment register so printing the game end message works correctly.
+  ; When running as a boot sector, cs=0, so this will reset es to 0. When
+  ; running as a DOS COM executable, the original value of es will be the same
+  ; as cs. (COM executables set each segment register to the same value)
+  ;
+  ; es = cs
+  mov di, cs
+  mov es, di
+
   mov ax, 0x1300
   mov dx, ((TextBuf.Height / 2) << 8) | (TextBuf.Width / 2 - GameOverStr.Len / 2)
-  ; es = 0
-  xor di, di
-  mov es, di
   int 0x10
 
 ;; Wait for restart key to be pressed, then restart game
@@ -548,12 +564,12 @@ WaitRestart:
 %warning Code is CodeSize bytes
 
 CodeEnd:
+%ifndef DOS
   ; Pad to size of boot sector, minus the size of a word for the boot sector
   ; magic value. If the code is too big to fit in a boot sector, the `times`
   ; directive uses a negative value, causing a build error.
   times (BootSector.Size - WordSize) - CodeSize db 0
 
   ; Boot sector magic
   dw 0xaa55
-
-;; Boot sector ends here
+%endif