Improve memory position handling

Makefile

@@ -1,8 +1,9 @@
 FLOPPY = 360
 
-RAM = 0x10000
-
+BDOS = 0xF000
 BDOSIMG = bdos.bin
+
+BIOS = 0xF800
 BIOSIMG = bios8086.bin
 
 PROGRAMS = $(patsubst %.asm,%.com,$(wildcard com/*.asm))
@@ -15,7 +16,7 @@ QEMU_ARGS = $(addprefix --option-rom ,$(ROMS))
 
 NASM = nasm
 NASM_ARGS = -s -Ilib -Plib/nasmcomp.asm -w-macro-params
-NASM_ARGS += -DRAM=$(RAM)
+NASM_ARGS += -DBDOS=$(BDOS) -DBIOS=$(BIOS)
 
 EMUL = utils/emul
 

bios/8086.asm

@@ -1,4 +1,4 @@
-ORG (RAM - 0x0800)
+ORG BIOS
 CPU 8086
 
 DISKCX: EQU 0x8
@@ -28,7 +28,8 @@ BOOT:
   DB "BIOS", 0
 
 WBOOT:
-  JMP (RAM - 0x1000)
+  XOR AX, AX
+  JMP BDOS
   RET
 
 ; CHAR I/O

boot/fat.asm

@@ -1,6 +1,5 @@
 %define segment   0x00100
 %define self      (0x7C00-(segment<<4)) ; 1 sector
-%define prog      (RAM-0x1000) ; 4K at the end for OS
 
 cpu 8086
 org self
@@ -58,7 +57,7 @@ main:
   ; Stack grows down from 64k
   mov ax, cs
   mov ss, ax
-  mov sp, prog
+  mov sp, BDOS
   mov ds, ax
   mov es, ax
 
@@ -67,7 +66,7 @@ main:
   mov ax, 1
   mov cx, 8
   mov dx, 0
-  mov bx, prog
+  mov bx, BDOS
 
 .loop:
   push ax
@@ -106,8 +105,8 @@ main:
 
   mov dl, [fdc.drv]
 
-  ; jump into kernel
-  jmp segment:(prog+0x0800)
+  ; jump into bios cold boot entry point
+  jmp segment:BIOS
 
 error:
   mov ax, bp