A.N.A.L.O.G. #15, January 1984
                     TO THE ATARI 5200
by Claus Buchholz
When Atari designed the 5200 "Supersystem" as a successor to
the  aging  2600  VCS, they made use of the state-of-the-art
hardware  they put into their 400/800 home computers.  As  a
result,  the systems are quite similar. The differences  are
great  enough, however, that transporting programs from  one
system to the other requires some effort.
The  5200  is  a  single-board machine with four  controller
jacks,  a cartridge slot, an I/O expansion connector  and  a
power/RF  cable.  It  shares the same VLSI  chips  with  the
400/800,  except  for  the 6520 PIA  (joystick/parallel  I/O
ports).  The  other chips (ANTIC, GTIA, POKEY and  the  6502
CPU)  are in the 5200, although some of their registers  are
in  different  memory locations than those in  the  400/800.
Also,   some  of  the  registers  serve  slightly  different
purposes.  Figure 1 is a block diagram of  the  5200.  Let's
look at each section of the hardware in turn.
When  a  specific register is mentioned in the article,  the
name  is  taken  from  the Hardware Manual  in  the  400/800
Technical  Reference  Notes. With the exceptions  listed  in
this article, the Hardware Manual applies also to the 5200.
                          6502 CPU.
Although  it is a standard 6502 from the programmer's  view,
Atari  has  reworked this chip to save four  support  chips.
Those  four  chips mediate access of the system bus  by  the
6502B  and ANTIC in the 400/800, but that function is  built
into the 6502C in the 5200. Atari also uses the 6502C in the
new XL series of computers.
                          16K RAM.
The  5200 contains 16K bytes of RAM addressed from $0000  to
$3FFF,  just as in an unexpanded 400. The memory circuit  is
nearly identical to that in the 400, except it uses 4516s, a
5V-only version of the 4116 16K-bit dynamic RAM.
The  5200  monitor program reserves locations $0000  through
$0018  and $0200 through $021B for shadows and RAM  vectors.
And, of course, page $01 is reserved for the 6502 stack. The
rest of the RAM is available to the cartridge program.
This is the same ANTIC chip used in the 400/800. Since ANTIC
shares  the address bus with the CPU and has no chip  select
input,  its registers reside in the same place in the 5200's
memory as in the 400/800's, page $D4.
The  5200  has  no System Reset key, so bit 5  of  NMIST  is
useless and the 5200's interrupt handler ignores it.
The GTIA and its registers perform the same functions in the
5200  and  400/800,  except as noted below.  The  registers,
however, reside at page $C0 in the 5200's memory, not at $D0
as in the 400/800.
The  trigger inputs, TRIG0 through TRIG3, are wired  to  the
controller ports, one to a port. The bottom button on either
side  of  the leftmost controller zeroes the TRIG0  register
when pressed, and likewise for the other ports.
The  bits  in  CONSOL,  the 400/800's  console  switch  port
(START, OPTION, SELECT and speaker), are used as outputs  in
the 5200, Bit 3, the 400/800's speaker control can still  be
toggled  in  the  5200  to produce  sounds  through  the  TV
speaker.   Bit   2  controls  the  pots  in   the   joystick
controllers. It must be set high to enable the pots.
Bits 1 and 0 select which controller port is to be active at
one  time. 00 selects port #1 (the leftmost), 01 selects #2,
10  selects  #3, and 11 selects #4. The trigger buttons  and
pots  are independent of this selection; it applies only  to
the keypads and top side buttons on the controllers.
POKEY's registers are all addressed at page $EB in the  5200
as  opposed  to  $D2  in  the  400/800.  Its  functions  are
unchanged, however, except for two.
The  eight  pot inputs used for paddles in the  400/800  are
wired  to  the 5200's controller ports, two to a port.  Each
controller  has an analog joystick, using one pot  to  sense
horizontal position and a second pot for vertical  position.
The  even pots (POT0-POT6) give the horizontal positions  of
range  from 1 to 228; the maximum readings are to the  right
for  the  horizontal pot and at the bottom for the  vertical
pot. Figure 2 shows the pinout for the 5200 controllers.
Pin  Function
1    Keypad -- right column
2    Keypad -- middle column
3    Keypad -- left column
4    Start, Pause, and Reset common
5    Keypad -- third row and Reset
6    Keypad -- second row and Pause
7    Keypad -- top row and Start
8    Keypad -- bottom row
9    Pot common
10   Horizontal pot (POT0, 2, 4, 6)
11   Vertical pot (POT1, 3, 5, 7)
12   5 volts DC
13   Bottom side buttons (TRIG0, 1, 2, 3)
14   Top side buttons
15   0 volts -- ground
The  keyboard scanning lines in the 400/800 are used in  the
5200  to read the keypad keys to the one controller that  is
selected  by  bits 1 and 0 in CONSOL. Only  four  lines  are
used,  though, so only bits 1 through 4 of KBCODE are valid.
Table  1  gives the keycode for each key on the  controller.
The top side buttons on the selected controller act like the
400/800 shift keys and also cause a BREAK-key interrupt,  if
that's  enabled.  Bit 0 of SKCTL, the debounce  enable  bit,
need not be set in the 5200.
Key     bits    Keypad code
none    0000    $FF
#       0001    $0B
0       0010    $00
*       0011    $0A
Reset   0100    $0E
9       0101    $09
8       0110    $08
7       0111    $07
Pause   1000    $0D
6       1001    $06
5       1010    $05
4       1011    $04
Start   1100    $0C
3       1101    $03
2       1110    $02
1       1111    $01
POKEY's serial I/O lines are used in the 5200, but they  are
wired  to  the  I/O expansion connector, an  edge  connector
hidden  behind  a small door in the rear of the  5200.  This
connector  allows for more hardware registers  addressed  at
page  $E0,  and for peripherals using the serial  port.  See
Figure  3  for  the pinout of this connector. Its  existence
demonstrates  Atari's  original plans  to  expand  the  5200
             Top                  Bottom
+5V DC                  1   36      +5V DC
Audio Out (2 port)      2   35      Not connected
Ground                  3   34      Ground
R/W Early               4   33      Not connected
Enable E0-EF            5   32      D7
D6                      6   31      D5
D4                      7   30      D3
D2                      8   29      D1
D0                      9   28      Ground
IRQ                     10  27      A0
Ground                  11  26      A1
Serial Data In          12  25      A2
Serial In Clock         13  24      A3
Serial Out Clock        14  23      A4
Serial Data Out         15  22      A5
Audio In                16  21      A6
A14                     17  20      A7
System Clock 01         18  19      A11
The 5200 has a 2K ROM on board which holds the character set
and  monitor program. The character set, which is  an  exact
copy of the 400/800's set, resides at pages $F8 through $FB,
and the monitor sits at $FC through $FF.
The  cartridge  ROM  can be 32K bytes long  and  resides  in
memory from $4000 to $BFFF. Figure 4 shows the pinout of the
cartridge  slot.  The  two interlock  connectors  are  wired
together  on  a  cartridge board. The 5200 uses  this  as  a
switch for the cartridge's power connections and as a  Reset
signal.  Therefore,  a cartridge may be  safely  removed  or
inserted while the 5200 is powered on.
D0                       1   36      Interlock
D1                       2   35      A11
D2                       3   34      A12
D3                       4   33      A10
D4                       5   32      A13
D5                       6   31      A9
D6                       7   30      Audio In (2 port)
D7                       8   29      A8
Enable 80-8F             9   28      Not Connected
Enable 40-7F             10  27      A7
Not Connected            11  26      +5V DC
Ground                   12  25      Ground
Ground                   13  24      Ground (Video In on 2 port)
Ground (System Clock
02 on 2 port)            14  23      Ground
A6                       15  22      A4
A5                       16  21      A3
A2                       17  20      A1
Interlock                18  19      A0
                  The 5200 monitor program.
The  1K  monitor program in ROM has three functions: (1)  to
initialize the system before running the cartridge  program,
(2) to service interrupts as they occur, and (3) to maintain
shadows  of  some important hardware registers. Recall  that
the  400/800 Operating System is 10K bytes long and performs
the  above  functions. It also provides peripheral handlers,
predefined  graphics  modes, a screen editor,  and  floating
point  math  routines. Those utilities do not exist  in  the
Table  2  shows  the RAM locations used by the  monitor  for
shadows and RAM vectors.
            Table 2. 5200 Monitor RAM Locations.
                     Page Zero Locations
$00       Shadow for IRQEN
$01       Real time clock (high byte)
$02       Real time clock (low byte)
$03       Critical  code  flag  (if  non-zero,  VBI  routine  is
$04       Attract mode timer/flag
$05       Shadow for DLISTL
$06       Shadow for DLISTH
$07       Shadow for DMACTL
$08-$10   Shadows for COLPMO through COLBK
$11-$18   Shadows for POT0 through POT7
                      Page Two Vectors
$200 Immediate IRQ vector
$202 Immediate VBI vector
$204 Deferred VBI vector
$206 DLI vector
$208 Keyboard IRQ vector
$20A Keypad routine continuation vector
$20C BREAK key IRQ vector
$20E BRK instruction IRQ vector
$210 Serial Input Data Ready IRQ vector
$212 Serial Output Data Needed IRQ vector
$214 Serial Output Finished IRQ vector
$216 POKEY Timer 1 IRQ vector
$218 POKEY Timer 2 IRQ vector
$21A POKEY Timer 4 IRQ vector
Upon   Reset,  the  6502  vectors  through  $FFFC   to   the
initialization routine. This routine performs the  following
1. Disable maskable interrupts, clear the 6502 decimal flag,
and set the stack pointed to $01FF.
2.  If  the cartridge address $BFFD contains $FF, then  jump
immediately   through  the  vector  at   $BFFE   (diagnostic
3.  Zero all hardware registers and page $00, set CHBASE  to
point  to the character set at $F8, and initialize the first
six RAM vectors starting at $0200.
4.  Set  up  the  Atari logo rainbow display. The  cartridge
title  (20 characters) and copyright year (2 characters)  in
ANTIC mode 7 display code are taken from cartridge addresses
$BFE8 through $BFFD.
5.  Enable  VBI (Vertical Blank Interrupt) and DLI  (Display
List Interrupt), and enable key scan.
6.  Wait four seconds, then jump through the vector at $BFFE
to the cartridge program.
When  the  6502 receives a non-maskable interrupt (NMI),  it
vectors  through  $FFFA  to the NMI handler.  The  following
steps take place:
1.  Check  NMIST  and strobe NMIRES to reset  the  interrupt
2.  If  a  DLI  is  pending, jump  through  the  DLI  vector
(initialized to point to the rainbow effect routine).
3.  If  a  VBI  is pending, jump through the  immediate  VBI
vector (initialized to point to the VBI routine).
4. Else, return from the interrupt (no System Reset).
A cartridge program can change these vectors to point to its
own  DLI  and  VBI  routines, if it must.  The  default  VBI
routine takes the following action.
1.  Push  A,  X, and Y onto stack, increment the  real  time
clock, and update the attract mode timer.
2.  If the critical code flag byte is non-zero, then pop  Y,
X, and A from the stack and return from the interrupt.
3. Update DLISTL, DLISTH, and DMACTL from their shadows.
4.  Maintain the attract mode flag and update the GTIA color
registers from their shadows.
5. Update the pot shadows from POT0 through POT7, and strobe
POTGO to start another pot scan.
6.  Jump  through  the deferred VBI vector  (initialized  to
point to the end-of-interrupt routine, which pops Y, X,  and
A, and returns from the interrupt).
If  maskable  interrupts  (IRQs)  are  enabled  and  one  is
received,  the 6502 vectors through $FFFE to an  instruction
which jumps through the immediate IRQ vector. That vector is
initialized to point to the IRQ routine, which performs  the
following tasks.
1. Push A and check IRQST.
2.  For each of the eight bits in IRQST, check for a pending
interrupt. If found, then clear the status bit, update IRQEN
from  its  shadow,  and  jump through  the  appropriate  IRQ
3.  If  no interrupt found, then push X and check for a  BRK
instruction interrupt. If found, then jump through  the  BRK
instruction IRQ vector.
4. Else, pop X and A and return from the interrupt.
The  only IRQ vector that is initialized is the keyboard IRQ
vector,  which  points  to  the keypad  read  routine.  That
routine does the following:
1. Push X and Y.
2. Read KBCODE and mask bits 1 through 4.
3. Convert to the keypad code given in Table 1, leaving that
code in A.
4.  Jump  through  the  keypad routine  continuation  vector
(initialized to point to the end-of-interrupt routine).
Comparing  the  5200's monitor vectors to the  400/800's  OS
vectors,   we   see   that  Atari  paid  no   attention   to
compatibility between the two. This further complicates  the
task of converting a program from one system to the other.
It  would  not be difficult, given the information  in  this
article,  to  write a program in two versions, one  for  the
400/800 and another for the 5200. Nor would it be difficult,
given  the  source code, to convert a finished program  from
the  5200  to the 400/800. The reverse is more difficult  if
the  program  takes  advantage of special  features  in  the
400/800  OS. Otherwise, the only task, aside from redefining
some  addresses,  is to convert the keyboard/joystick  input
routines from one system to the other.
I  acquired the information in this article by dissecting  a
5200  and  disassembling its ROM. The 400/800 schematics  in
the Hardware Manual were quite helpful. It is interesting to
note  the  difference between the two machines and to  guess
Atari's   motives  for  the  design  differences.  But   the
similarities  grossly outweigh the differences,  so  that  a
5200  program can be developed and almost entirely  debugged
before  testing  on a 5200. With the addition  of  an  EPROM
burner,  a 400/800 can be a powerful development system  for
5200  programs. An adventuresome hacker can even bypass  the
EPROM  by  putting dual-port RAM on the cartridge board  and
downloading  programs  from the 400/800  development  system
into the 5200 for testing.
A.N.A.L.O.G. #16, February 1984
5200 Article Update. (ANALOG #15)
Newer  releases of the 5200 incorporate some minor  hardware
changes.  Controller  ports 3 and 4  have  been  eliminated,
making  POT4 through POT7, TRIG2, TRIG3, and bit 1 of CONSOL
useless.  A  few of the connector pins have been  redefined.
Pin  2  of  the I/O expansion connector now carries  POKEY's
Audio Out signal. Three pins on the cartridge connector have
changed  to  accomodate  the new 2600  adapter.  The  system
clock, 02, is output on pin 14, isolated through a diode. An
alternate  video  input is taken from pin  24  and  is  also
isolated through a diode. Pin 30 provides an alternate audio
There  is space on the newer boards for circuitry for a  PAL
(European TV standard) version of the 5200. Also, on  power-
up,  the  monitor  program checks for  the  PAL  version  by
examining  the  GTIA  register  PAL  after  step  2  of  the
initialization routine. It also checks the cartridge program
for PAL compatibility. The byte at $BFE7 should read $02  if
compatible, or $00 if not. This is the only important change
to  the  monitor program. There are some additional hardware
changes,  but none affects the machine's operation from  the
programmer's view.
Claus Buchholz
Greenwich, CT