Comparison of RT-11 and PDP-11 Unix binary code headers
                 =======================================================

-------+---------------------------------------+------------------------------------------+
 File  |                                       |                                          |
Offset |  RT-11                                |  Unix                                    |
=======+=======================================+==========================================+
       | Normally ignored by RT-11:            | Read by Unix when code executed:         |
000000 |  VIR in Radix-50                      |  amagic:  Magic word, also Branch opcode |
000002 |  Virtual high limit                   |  atext:   Size of code                   |
000004 |  Job definition word ($JSX)           |  adata:   Size of initialised data       |
000006 |  Reserved                             |  abss:    Size of uninitialised data     |
000010 |  Reserved                             |  asyms:   Size of symbol table           |
000012 |  Reserved                             |  aentry:  Entry address                  |
000014 |  BPT trap PC                          |  aunused: Not used                       |
000016 |  BPT trap PSW                         |  aflag:   Relocation info                |
       |                                       +------------------------------------------+
000020 |  IOT trap PC                          | Start of Unix code, loaded to 000000     |
000022 |  IOT trap PSW                         |                                          |
000024 |  Reserved                             |                                          |
000026 |  Reserved                             |                                          |
000030 |  Reserved                             |                                          |
000032 |  Overlay definition word              |                                          |
000034 |  Trap vector PC (TRAP)                |                                          |
000036 |  Trap vector PSW (TRAP)               |                                          |
-------+---------------------------------------+                                          |
       | Read by RT-11 when code executed:     |                                          |
000040 |  Entry address                        |                                          |
000042 |  Initial stack pointer                |                                          |
000044 |  Job Status Word                      |                                          |
000046 |  USR swap address                     |                                          |
000050 |  Program high limit                   |                                          |
000052 |  Reserved                             |                                          |
000054 |  Reserved                             |                                          |
000056 |  Reserved                             |                                          |
-------+---------------------------------------+                                          |
       | Normally ignored by RT-11:            |                                          |
000060 |  Reserved                             |                                          |
000062 |  Reserved                             |                                          |
000064 |  Overlay handler address              |                                          |
       |                                       |                                          |
000066 |  Window definition blocks             |                                          |
000070 |                                       |                                          |
  to   |  Reserved                             |                                          |
000356 |                                       |                                          |
-------+---------------------------------------+                                          |
       | Read RT-11 when code executed:        |                                          |
000360 |  Bitmap area                          |                                          |
  to   |   Bit set for each 512-byte block to  |                                          |
000377 |   be loaded                           |                                          |
-------+---------------------------------------+                                          |
000400 | Not loaded, usually used as initial   |                                          |
       | stack area with sp=001000             |                                          |
-------+---------------------------------------+                                          |
001000 | Start of RT-11 code, loaded to 001000 |                                          |
       |                                       |                                          |
       |                                       +------------------------------------------+
       |                                       | atext:           Initialised data        |
-------+---------------------------------------+------------------------------------------+
       |                                       | atext+adata:     Uninitialised data      |
       |                                       |                  End of loaded code      |
-------+---------------------------------------+------------------------------------------+
       | Program high limit                    | atext+adata+bss: Symbol data             |
-------+---------------------------------------+------------------------------------------+
                                               | atext+adata+abss+asyms: End of file



                        Sample assembler code
                        ---------------------

; Unix header at &0000-&000F (&o0000 to &o0017)
; ---------------------------------------------
.word  &0107			; Magic number &o000407
.word  CodeEnd-UxStart		; size of text
.word  DataEnd-CodeEnd		; size of initialised data
.word  ObjectEnd-DataEnd	; size of uninitialised data
.word  &0000			; size of symbol data
.word  UxEntry-UxStart		; entry point
.word  &0000			; not used
.word  &0001			; no relocation info

; &o0020 : Unix loads code from here onwards to memory at &o0000
; --------------------------------------------------------------
.UxStart

; RT-11 startup info at &0020 (&o0040)
; ------------------------------------
.blkb  32-$			; Pad to &o40
.word  RtEntry			; RT-11 entry point
.word  RtStackTop		; RT-11 top of stack
.word  &0000			; Job Status Word
.word  &0000			; USR swap address
.word  ObjectEnd		; Program high memory limit

; RT-11 code bitmap
; -----------------
.blkb  240-$			; Pad to bitmap
.byte  %11000000		; First two 512-byte blocks used
.byte  %00000000
.blkb  256-$			; Pad to end of bitmap

; RT-11 stack space
; -----------------
.align &200			; Pad to end of RT-11 header
.RtStackTop			; RT-11 usually has stack here

; &o1000 : RT-11 loads code from here onwards into memory at &o1000
; -----------------------------------------------------------------
RtEntry:
; Code entered by RT-11

UxEntry:
; Code entered by Unix

CodeEnd:			; End of code for Unix header
; Initialised data here

DataEnd:			; End of initialised data for Unix header
; Uninitialised data here, Unix sets all to zero on loading

ObjectEnd:			; End of entire object
; Running RT-11 programs can move this limit upwards with the SETTOP call.
; Running Unix programs can move this limit upwards with the brk() call.