1200XL & portb

[+Larry]

I'm still not very knowledgeable about 1200XL's. Since they don't have built-in BASIC and they don't have expanded memory, do the 1200XL's use portb in some fashion like the other XL/XE models? IIRC, you can add built-in BASIC along with the 800XL OS, but that's about all I remember.

 

-Larry

[yorgle]

Not sure what you mean by "expanded memory" but they do have 64k ram same as the 800xl.

[Urchlay]

There's no built-in BASIC, but the 1200XL still uses the same PORTB bits as the 800XL does for enabling/disabling the self-test ROM and the main OS ROM (to get at the RAM under it, since the 1200XL has 64K like the 800XL). Two of the other bits control the L1 and L2 LEDs that only exist on the 1200XL... a lot of 256K memory upgrades for the 1200 leave the LEDs connected, so you can actually watch the LEDs and see when extended RAM is being accessed.

 

If you have the revised edition of "Mapping the Atari", Appendix 12 explains exactly what PORTB bits the 1200XL uses... I'd point you to the web version at atariarchives.org, but it seems to be down at the moment. It's short enough I might as well paste the relevant stuff here...

 

You can disable the ROM between 49152-53247 ($C000-$CFFF)

and 55296-65535 ($D800-$FFFF) by setting bit 0 to 0 (the ROM

area becomes RAM; note that the area between $D000 and

$D7FF remains intact). However, unless another OS has been

provided, the system will crash at the next interrupt (1/60 sec-

ond later!), so you need to disable the interrupts first.

 

Bit 1 controls BASIC; if 0, BASIC is enabled, if 1, it is disabled

and the 8K RAM space enabled for program use. If you disable

BASIC within a BASIC program, you lock up. Disable BASIC dur-

ing a boot operation by holding down the OPTION key.

 

Bits 2 and 3 control the 1200XL LEDs; 0 means on, 1 means off.

LED 1 means the keyboard is disabled; LED 2 means the inter-

national character set is selected. In the 130XE, these bits are

used for bank switching 16K blocks of RAM. The 130XE allows

you to use the extra memory as video memory or program/

data memory. See the section on memory management in the

13OXE at the end of this chapter.

 

Bits 4-6 are reserved (unused) in the XL and 65XE. Bits 4 and 5

in the 13OXE are used to enable bank switching (see below).

 

Bit 7 controls the RAM region 20480-22527 ($5000-$57FF), nor-

mally enabled 1). When disabled 0), the OS ROM in that area is

enabled and access provided to the self-test code moved from

53248-55295 ($D000-$D7FF).

 

So in the 1200XL, bits 0 and 7 work the same as in an 800XL. Bit 1 does nothing (no BASIC to disable), bits 2 and 3 are the LEDs, and 4-6 do nothing.

[+Guitarman]

I'm still not very knowledgeable about 1200XL's. Since they don't have built-in BASIC and they don't have expanded memory, do the 1200XL's use portb in some fashion like the other XL/XE models? IIRC, you can add built-in BASIC along with the 800XL OS, but that's about all I remember.

 

-Larry

 

Adding the 800XL OS and BASIC is an easy mod. I have done it to almost all of my 1200XL's. Here is a post I added a while back.

 

1200XL OS/BASIC Upgrade

[Urchlay]

Adding the 800XL OS and BASIC is an easy mod. I have done it to almost all of my 1200XL's. Here is a post I added a while back.

 

...not having to hold down Option at boot is one of the best things about using a 1200XL, though...

[+Guitarman]

Adding the 800XL OS and BASIC is an easy mod. I have done it to almost all of my 1200XL's. Here is a post I added a while back.

 

...not having to hold down Option at boot is one of the best things about using a 1200XL, though...

 

Just burn an OS with the option reversed!!

[+Larry]

Thanks, guys!

 

Excellent info from Mapping... and the Basic & OS Mod.

 

"Expanded" as I used it meant more than 64K, but non-linear. "Extended" is as with the 65C816 -- linearly addressable beyond 64K.

 

I appreciate the responses!

 

-Larry