Atari 800 memory query

[spacedmonkeys]

I have acquired a few Atari 8-bit computers lately (I have a retro thirst that needs satisfying).

 

Now, I know there is a way to detect the amount of memory using basic command PRINT FRE(0), and the amount returned is 37902.

 

I have tried this on a 400 with 48k, and two 800's both with 48k.

 

I have one 800 however which is showing 37890 bytes free rather than 37902.

 

What would cause this, faulty? or a newer/older OS ROM which takes up slightly more memory?

[Paul Westphal]

Try alternating the ram cards. Pull them all out and test them in the first slot ( behind the O.S. card ) one by one, by typing ?fre(0), and see what you get.

[Guest]

a difference in size between

NTSC - OS-A,

NTSC -OS-B,

NTSC -OS-B (ver 2) and

PAL- OS-A ?

 

here's the roms (from another thread on Atariage)

OS Rev A (1979) (Atari) (PAL) (400-800).rom

OS Rev B (1981) (Atari) (NTSC) (400-800) V1.rom

OS Rev B (1981) (Atari) (NTSC) (400-800) V2.rom

OS Rev A (1979) (Atari) (NTSC) (400-800).rom

Edited August 4, 2015 by Guest

[flashjazzcat]

Is this not related to the Rev. B BASIC bug which increases the size of the tokenised program between every SAVE (thus decreasing the amount of free RAM)? Although that would make no sense if you're using the same BASIC cart in all machines.

[+DrVenkman]

Is this not related to the Rev. B BASIC bug which increases the size of the tokenised program between every SAVE (thus decreasing the amount of free RAM)? Although that would make no sense if you're using the same BASIC cart in all machines.

 

 

Nope. A 48K Atari running standard OS-B and any version of Atari BASIC should always return 37,902 bytes of free memory from ? FRE(0)

[tep392]

I like puzzles. fre(0) is calculated from memtop (02e5/02e6) minus topstk (0090/0091). To start tracking down the culprit, you should PEEK at these memory locations and see which is different on the machine in question. Then we can start tracing back to the source of the difference. On the machines that show 37902 free, you should get these results

memtop

peek(741) = 31

peek(742) = 156

topstk

peek(144) = 17

peek(145)=8

 

(156*256+31) - (8*256+17) = 37902

 

What do you get on the machine that shows 37890 free?

[spacedmonkeys]

Ok so I've reseated all the ram cards and now both 800s are showing the correct result from fre(0) ! So still a mystery but one I can't replicate now !!