Add support for 360K disk images as well

Makefile

@@ -1,3 +1,5 @@
+FLOPPY = 1440
+
 .SUFFIXES:
 .SUFFIXES: .bin .asm
 
@@ -5,15 +7,15 @@ all: nor86.img
 
 nor86.img: bootsect.bin kernel.bin CC0 README.md
 	rm -f $@
-	mkdosfs -C $@ 1440
+	mkdosfs -C $@ $(FLOPPY)
 	rw -i bootsect.bin -o $@
 	mcopy -i $@ CC0 ::
 	mcopy -i $@ README.md ::
 	mcopy -i $@ kernel.bin ::
 
 .asm.bin:
-	nasm -fbin -o $@ $<
-	nasm -MD $@.d -fbin -o $@ $<
+	nasm -fbin -d F$(FLOPPY) -o $@ $<
+	nasm -MD $@.d -d F$(FLOPPY) -fbin -o $@ $<
 
 clean:
 	rm -f *.bin *.img *.bin.d

README.md

@@ -3,3 +3,11 @@ Nor86 operating system
 
 Nor86 is a real-mode operating system targeting IBM PC compatibles with 64K
 or more RAM. It is inspired by [EttinOS](https://ahti.space/git/crazyettin/EttinOS).
+
+Floppy sizes
+------------
+
+You can build either a 1440K or a 360K floppy image from Nor86 sources, the
+default being 1440K. To build a 360K floppy instead, run:
+
+    make FLOPPY=360

bootsect.asm

@@ -4,6 +4,7 @@ org 0x7c00
 jmp short _code
 nop
 
+%ifdef F1440
 ; 1440K floppy
 ; BPB
 oemidentifier     db "nor86   "
@@ -20,6 +21,27 @@ heads             dw 2
 hiddensectors     dd 0
 totalsectorslarge dd 0
 
+%elifdef F360
+; 360K floppy
+; BPB
+oemidentifier     db "nor86   "
+byterpersector    dw 512
+sectorspercluster db 2
+reservedsectors   dw 1
+fats              db 2
+rootdirentries    dw 112
+totalsectors      dw 720
+mediadescription  db 0xfd
+sectorsperfat     dw 2
+sectorspertrack   dw 9
+heads             dw 2
+hiddensectors     dd 0
+totalsectorslarge dd 0
+
+%else
+%error "No valid floppy format specified, specify -d F1440 or -d F360"
+%endif
+
 ; EBPB
 drivenumber       db 0 ; useless on-disk, used as a variable
 reserved          db 0 ; winnt flags