[syslinux] modifying initrd before high mem load

[Amos Waterland]

Hi, I have an interesting question for you.  Given the output of
scramble(initrd) = S, I am interested in having [iso|sys|pxe]linux
perform unscramble(S) = initrd before they load it into high memory.

As a simple example, suppose scramble() were simply a XOR with a
constant value of the gzip magic number at the front of the initrd
image.  I would like to insert the following, but I can't seem to figure
out how to correctly address the data pointed to by SI.  Any help would
be greatly appreciated.

  ;
  ; Load RAM disk into high memory
  ;
  ; Need to be set:
  ;       su_ramdiskat    - Where in memory to load
  ;       su_ramdisklen   - Size of file
  ;       SI              - initrd filehandle/cluster pointer
  ;
  loadinitrd:
                  push es                         ; Save ES on entry
                  mov ax,real_mode_seg
                  mov es,ax
                  mov edi,[es:su_ramdiskat]       ; initrd load address
+ unscrambleinitrd:
+                 xor [si:0],constant_xor_value

                  push si
                  mov si,crlfloading_msg          ; Write "Loading "
                  call cwritestr
                  mov si,InitRDCName              ; Write ramdisk name
                  call cwritestr
                  mov si,dotdot_msg               ; Write dots
                  call cwritestr
                  pop si

                  mov eax,[es:su_ramdisklen]
                  call load_high                  ; Load the file

                  call crlf
                  pop es                          ; Restore original ES
                  ret