Studer A810 PHG Firmware Analysis
==================================
Version:  4086 (week 40, 1986)
Author:   PHG, Studer Regensdorf (Switzerland)
Date:     June 5, 1986
Base:     v2586 (most likely, identical reset structure)
Files:    1.bin=IC14(E000), 2.bin=IC12(C000), 3.bin=IC10(A000)


MEMORY MAP
----------
Identical to v1183 and v2586:
  IC10 (3.bin): $A000-$BFFF
  IC12 (2.bin): $C000-$DFFF
  IC14 (1.bin): $E000-$FFFF

Reset: $FFE8 -> SEI + JMP $A000  (same as v2586)


CONFIRMED FUNCTIONAL CHANGE: LOC+REC
--------------------------------------

v1183 transport modes (11 total, pointer table at $C280):
  [0]  TAPE OUT    [1]  TAPE LOAD   [2]  STOP
  [3]  REWIND      [4]  FORWARD     [5]  PLAY
  [6]  RECORD      [7]  EDIT        [8]  LOCATE
  [9]  LOC+PLAY    [10] TAPE DUMP

PHG transport modes (12 total, pointer table at $CC44):
  [0]  TAPE OUT    [1]  TAPE LOAD   [2]  STOP
  [3]  REWIND      [4]  FORWARD     [5]  PLAY
  [6]  RECORD      [7]  EDIT        [8]  LOCATE
  [9]  LOC+PLAY    [10] TAPE DUMP
  [11] LOC+REC     <-- NEW

LOC+REC = Locate + Record:
  The machine spools to a pre-set locate point and then
  automatically starts recording. Functionally equivalent to
  an electronic punch-in at a fixed tape position.
  Useful for repeated recording takes at exactly the same point.

v2586 does NOT have this mode (also 11 modes, no LOC+REC string).
PHG therefore added LOC+REC as a new feature on top of v2586.


TRANSPORT MODE DISPATCH ROUTINE (PHG $CB4E)
--------------------------------------------

The routine at $CB61-$CB7E is the core of mode selection:

  $CB61  LDX  #$CC44    ; load pointer to string table
  $CB64  ABX             ; add mode index (B register)
  $CB65  PSHA            ; save A
  $CB66  LDAA $3B        ; load current state
  $CB68  CMPA #$10       ; mode $10?
  $CB6A  BEQ  $CB72      ; yes -> special case
  $CB6C  CMPA #$12       ; mode $12?
  $CB6E  BEQ  $CB72      ; yes -> special case
  $CB70  BRA  $CB7E      ; no -> normal path
  $CB72  LDAA $30        ;
  $CB74  ORAA $32        ; combine status registers
  $CB76  ANDA #$10       ; test bit 4 (record ready?)
  $CB78  BEQ  $CB7E      ; not ready -> normal path
  $CB7A  INX             ; advance 4 bytes in table =
  $CB7B  INX             ; select LOC+REC string
  $CB7C  INX             ; instead of LOC+PLAY
  $CB7D  INX
  $CB7E  PULA
  $CB7F  LDX  $00,X      ; dereference pointer to string
  $CB81  JSR  $DB34      ; display string

Interpretation: in mode $10 or $12, if bit 4 of the combined
status ($30 OR $32) is set, the display pointer advances 4 bytes --
which selects "LOC+REC" instead of "LOC+PLAY". This is a
state-dependent display: the same button shows a different label
depending on whether the machine is in record-ready state.


ADDITIONAL OBSERVATIONS
------------------------

1. PHG has 11920 disassembly lines vs 11556 for v2586 (+364).
   The extra code is distributed throughout, not in a contiguous block.

2. The full version string includes location and exact date:
   "A810 SOFTWARE VERSION 4086 REVIDED AND IMPROVED BY PHG
    . REGENSDORF,  5.06.1986"
   "REVIDED" is a misspelling of "REVISED" -- likely written by a
   non-native English speaker (Swiss/German).

3. Interrupt vectors differ from v2586:
              v2586    PHG
   TRAP:      $D4DB    $D7F4
   SCI:       $CF69    $D2A7
   TOF:       $D377    $D6B5
   OCF:       $D45A    $D798
   ICF:       $D45A    $D798
   IRQ:       $D99B    $DCFC
   SWI:       $D45B    $D799
   All handlers are relocated but structurally equivalent.

4. PHG retains the "READING RS232" and "PROGRAM LOADED" strings
   (also present in v1183) -- the serial program loading function
   is preserved.

5. v2586 has "INPUT FORMAT ERROR !" -- PHG does not.
   This error message may have been handled differently or removed.


CONCLUSION
-----------
PHG used v2586 (or a closely related version) as a base and
primarily added the LOC+REC transport mode. This is an operationally
meaningful extension for studio use where repeated punch-in recordings
at a fixed locate point are required.
The change is limited in scope but functionally significant.

The large byte-level differences (~97%) are the result of code
relocation (refactoring) rather than a wholesale rewrite of all
functions. The overall program structure and hardware interface
remain consistent with the Studer v1183 and v2586 firmware lineage.