Do enough of the boot stub to produce an EXE header and enter DPMI protected

mode.

plat/msdos386/boot.s

@@ -12,6 +12,130 @@
 
 .sect .text
 .use16
+exe_header:
+    .data2 0x5a4d               ! magic number
+    .data2 0                    ! number of bytes in last loadable page
+    .data2 exe_text_paras       ! size of .exe, in pages
+    .data2 0                    ! number of relocation entries
+    .data2 0                    ! start of loadable area, in 16-byte paragraphs
+    .data2 exe_ram_paras        ! required RAM size, in 16-byte paragraphs
+    .data2 0                    ! maximum RAM siz, in 16-byte paragraphse
+    .data2 0                    ! initial SS, relative to program
+    .data2 exe_stack            ! initial SP
+    .data2 0                    ! checksum (ignored)
+    .data2 exe_start            ! initial IP
+    .data2 0                    ! initial CS, relative to program
+    .data2 0                    ! offset of relocation table
+    .data2 0                    ! overlay number
+
+dpmi_edi    = 0x00
+dpmi_esi    = 0x04
+dpmi_ebp    = 0x08
+dpmi_ebx    = 0x10
+dpmi_edx    = 0x14
+dpmi_ecx    = 0x18
+dpmi_eax    = 0x1c
+dpmi_flags  = 0x20
+dpmi_es     = 0x22
+dpmi_ds     = 0x24
+dpmi_fs     = 0x26
+dpmi_gs     = 0x28
+dpmi_ip     = 0x2a
+dpmi_cs     = 0x2c
+dpmi_sp     = 0x2e
+dpmi_ss     = 0x30
+
+dpmi_ps     = 0x32 ! WORD: protected register segment
+dpmi_rs     = 0x34 ! WORD: real register segment
+dpmi_psp    = 0x36 ! WORD: PSP segment
+dpmi_switch = 0x38 ! DWORD: far pointer of pmode switch routine
+
+    .seek 0x3c
+exe_start:
+    ! On entry, DS=ES=PSP. Make DS=CS, so we're running in tiny mode.
+
+    push cs
+    pop ds
+    mov (dpmi_rs), ds
+    mov (dpmi_psp), es
+
+    ! Initialise DPMI.
+
+    mov ax, 0x1687
+    int 0x2f
+    or ax, ax
+    jnz no_dpmi
+    mov (dpmi_switch+0), di     ! write back PMODE switch routine
+    mov (dpmi_switch+2), es
+    or si, si                   ! do we need a DPMI private area?
+    jz 1f
+
+    mov bx, si
+    movb ah, 0x48
+    int 0x21                    ! allocate memory from DOS
+    mov es, ax                  ! data area segment -> es
+1:
+    ! Switch to protected mode.
+
+    mov ax, 1                   ! 32-bit app
+    callf (dpmi_switch)
+    jc no_pmode
+
+    mov (dpmi_edx), go_msg
+    mov ax, (dpmi_rs)
+    mov (dpmi_ds), ax
+    mov (dpmi_eax+1), 9
+    call int21
+    mov ax, 0x4c00
+    int 0x21
+
+    ! Simulate DOS interrupt bx.
+int21:
+    mov bx, 0x21
+callint:
+    xor ax, ax
+    mov (dpmi_ss), ax           ! zero stack: DPMI host allocates one.
+    mov (dpmi_sp), ax
+    push ds
+    pop es
+    mov di, ax
+    mov ax, 0x300
+    int 0x31                    ! simulate DOS interrupt
+    push cs
+    pop ds
+    ret
+
+no_pmode: ! Could not switch to protected mode.
+    mov dx, no_pmode_msg
+    jmp exit_with_error
+
+no_dpmi:
+    mov dx, no_dpmi_msg
+    ! fall through
+
+! Displays the message in dx and exits.
+exit_with_error:
+    movb ah, 9
+    int 0x21                    ! print $-terminated string
+    mov ax, 0x4cff
+    int 0x21                    ! terminate with error code al
+
+    .use32
+start_32bit:
+
+go_msg:
+    .ascii "Go!$"
+no_pmode_msg:
+    .ascii "Couldn't switch to protected mode$"
+no_dpmi_msg:
+    .ascii "No DPMI$"
+
+exe_top:
+
+exe_stack = exe_top + 512
+exe_text_paras = [exe_top - exe_header + 511] / 512
+exe_ram_paras = [exe_stack - exe_top + 15] / 16
+
 
 #define STACK_BUFFER 128 /* number of bytes to leave for stack */
 
@@ -30,8 +154,6 @@ begtext:
 
     mov dx, bad_sys_msg
 dos_msg:
-	movb ah, 9
-	int 0x21
     ret
 
 ok_sys:
@@ -168,3 +290,6 @@ no_room_msg: .ascii 'No room$'
 .comm .trppc, 4
 .comm .ignmask, 4
 .comm _errno, 4
+
+! vim: ts=4 sw=4 et ft=asm
+