Intertan DT-88 restoration (Late Tandy subsidiary 8088 clone)

Reply 20 of 29, by Predator99

Posted on 2022-03-09, 22:46

Predator99 Offline

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So I am quite sure the sub at C538 is for reading data from CMOS (Port 03E0 / 03E1). The data to read is given in AL when it s called:

1F000:C51E BAE003        MOV	DX,03E0                            
2F000:C521 EE            OUT	DX,AL                              
3F000:C522 EB00          JMP	C524                               
4F000:C524 EB00          JMP	C526                               
5F000:C526 42            INC	DX                                 
6F000:C527 EC            IN	AL,DX                              
7F000:C528 C3            RET	                                   
8
9F000:C529 BAE003        MOV	DX,03E0                            
10F000:C52C EE            OUT	DX,AL                              
11F000:C52D 86C4          XCHG	AL,AH                              
12F000:C52F EB00          JMP	C531                               
13F000:C531 EB00          JMP	C533                               
14F000:C533 42            INC	DX                                 
15F000:C534 EE            OUT	DX,AL                              
16F000:C535 86C4          XCHG	AL,AH                              
17F000:C537 C3            RET	                                   
18
19F000:C538 9C            PUSHF	                                   
20F000:C539 FA            CLI	                                   
21F000:C53A 52            PUSH	DX                                 
22F000:C53B E8E0FF        CALL	C51E                               
23F000:C53E 50            PUSH	AX                                 
24F000:C53F EB07          JMP	C548                               
25F000:C541 9C            PUSHF	                                   
26F000:C542 FA            CLI	                                   
27F000:C543 52            PUSH	DX                                 
28F000:C544 50            PUSH	AX                                 
29F000:C545 E8E1FF        CALL	C529                               
30
31F000:C548 B00D          MOV	AL,0D                              
32F000:C54A E8D1FF        CALL	C51E                               
33F000:C54D 58            POP	AX                                 
34F000:C54E 5A            POP	DX                                 
35F000:C54F 9D            POPF	                                   
36F000:C550 C3            RET

It is called around 20 times. I think this is where it is called from to check for floppy etc:

1F000:E2A2 B00E          MOV	AL,0E              Post Code 0E               
2F000:E2A4 E680          OUT	80,AL                              
3F000:E2A6 F7C50020      TEST	BP,2000            Check if CMOS inoperational               
4F000:E2AA B00D          MOV	AL,0D                              
5F000:E2AC 756F          JNZ	E31D               CMOS inop? Go to POST code 10                
6
7F000:E2AE E887E2        CALL	C538          AL=0D                     
8F000:E2B1 0AC0          OR	AL,AL                              
9F000:E2B3 B80E0E        MOV	AX,0E0E                            
10F000:E2B6 E87FE2        CALL	C538          AL=0E                    
11F000:E2B9 780B          JS	E2C6                               
12
13F000:E2BB 83CD08        OR	BP,+08      Set BP floppy error                       
14F000:E2BE 0CC4          OR	AL,C4                              
15F000:E2C0 EB42          JMP	E304                               
16
17F000:E2C2 90            NOP	                                   
18F000:E2C3 E96214        JMP	F728                               
19
20F000:E2C6 0AC0          OR	AL,AL                              
21F000:E2C8 7833          JS	E2FD                               
22
23F000:E2CA 50            PUSH	AX                                 
24F000:E2CB 33C9          XOR	CX,CX                              
25F000:E2CD 33D2          XOR	DX,DX                              
26F000:E2CF B81010        MOV	AX,1010                            
27F000:E2D2 E863E2        CALL	C538          AL=10                     
28F000:E2D5 8AC8          MOV	CL,AL                              
29F000:E2D7 03D1          ADD	DX,CX                              
30F000:E2D9 FEC4          INC	AH                                 
31F000:E2DB 8AC4          MOV	AL,AH                              
32F000:E2DD 3C1E          CMP	AL,1E                              
33F000:E2DF 7402          JZ	E2E3                               
34
35F000:E2E1 EBEF          JMP	E2D2                               
36
37F000:E2E3 FEC4          INC	AH                                 
38F000:E2E5 E850E2        CALL	C538                               
39F000:E2E8 86C4          XCHG	AL,AH                              
40F000:E2EA E84BE2        CALL	C538                               
41F000:E2ED 3BC2          CMP	AX,DX                              
42F000:E2EF 58            POP	AX                                 
43F000:E2F0 7504          JNZ	E2F6                               
44
45F000:E2F2 0BD2          OR	DX,DX                              
46F000:E2F4 750C          JNZ	E302                               
47
48F000:E2F6 0C44          OR	AL,44                              
49F000:E2F8 83CD10        OR	BP,+10    Set BP checksum error                        
50F000:E2FB EB07          JMP	E304                               
51
52F000:E2FD 83CD20        OR	BP,+20    Set BP display error               
53F000:E300 EB02          JMP	E304                               
54
55F000:E302 249B          AND	AL,9B                              
56
57F000:E304 50            PUSH	AX                                 
58F000:E305 B00F          MOV	AL,0F                              
59F000:E307 E680          OUT	80,AL        		Post Code 0F
60F000:E309 B014          MOV	AL,14

…Show last 11 lines

61F000:E30B E82AE2        CALL	C538                               
62F000:E30E D0E8          SHR	AL,1                               
63F000:E310 58            POP	AX                                 
64F000:E311 7205          JB	E318                               
65F000:E313 83CD20        OR	BP,+20                             
66F000:E316 0C20          OR	AL,20                              
67F000:E318 86C4          XCHG	AL,AH                              
68F000:E31A E824E2        CALL	C541                               
69
70F000:E31D B010          MOV	AL,10                              
71F000:E31F E680          OUT	80,AL               Post Code 10

Reply 21 of 29, by Predator99

Posted on 2022-03-09, 23:20

Predator99 Offline

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I think chances are high if you take a program like ATSETUP.COM and replace all references to port 70/71 with 03E0 / 03E1 you will get something useful as this seems to be the same RTC than in a AT with a different port?

Reply 22 of 29, by Deunan

Posted on 2022-03-10, 00:12

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I've seen that A000 check in other BIOSes as well (early 286 machine). Could be a ROM check, or some clever way to put executable code in video memory to survive hardware reset? That would depend on how that memory is refreshed though. In any case, I have never seen a ROM at A000 so I don't know what this is for.

BASIC ROM usually lives in lower 32k half of the F000 segment, though perhaps not starting at F000 (some sources say F600 to FDFF). Point is, to use these extra ROM sockets on the mobo the BIOS image should be only 32k I think, unless there is a way to rejumper them to E000. Might be possible with some HW mod - depends on the mobo? I don't know much about the mapping of these sockets to actual address space, and I don't have XT machine to check.

BTW on AT machines the most significant bit of port 70 is intercepted and not used for addressing CMOS (so there can only be 128 bytes in NVRAM without extra tricks like page swapping). The MSb is instead used as mask for NMI. So NMI is in fact maskable interrupt, albeit with extra hardware to do it outside the CPU. This is probably different for XT and also not used in that way on any CMOS NVRAM that is mapped on different address.

Reply 23 of 29, by weedeewee

Posted on 2022-03-10, 14:19

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Predator99 wrote on 2022-03-09, 23:20:

I think chances are high if you take a program like ATSETUP.COM and replace all references to port 70/71 with 03E0 / 03E1 you will get something useful as this seems to be the same RTC than in a AT with a different port?

Is there any indication of a checksum being calculated / stored / verified in the rtc nvram ?

Right to repair is fundamental. You own it, you're allowed to fix it.
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Reply 24 of 29, by Predator99

Posted on 2022-03-10, 19:09

Predator99 Offline

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Can you try this? I expect at least the clock to be working...

Filename: Set3e0.zip
File size: 5.35 KiB
Downloads: 46 downloads
File license: Fair use/fair dealing exception

Reply 25 of 29, by Deksor

Posted on 2022-08-03, 22:18

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Bump, I didn't see your latest message @Predator99. However I come with great news !
Thanks to a discord user named Makefile, I finally found a way to configure my bios !

Just as I was thinking at the beginning, there is indeed a BIOS setup hidden in there !!

To access it, it's very simple.
Run DOS' debug.exe and input g=f000:a100

And voilà !

So if you ever come across this beautiful computer, the magic word is

1g=f000:a100

Now I wonder if it's possible to restore the expected functionality from the BIOS (press del to enter CMOS at POST when everything's ok and press F1 when there's an error) by patching the ROM, it would make things far simpler.

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 26 of 29, by Predator99

Posted on 2022-08-05, 18:56

Predator99 Offline

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Oh cool!

Can you try my program anyway? 😉

I think then it should also be possible to call the Setup with a hotkey. Maybe I will take a look again if the mentioned adress is called somewhere within the BIOS.

Reply 27 of 29, by Skip94

Posted on 2023-10-09, 06:36

Skip94 Offline

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Predator99 wrote on 2022-08-05, 18:56:

Oh cool!

Can you try my program anyway? 😉

I think then it should also be possible to call the Setup with a hotkey. Maybe I will take a look again if the mentioned adress is called somewhere within the BIOS.

Predator99
I'm currently having some issues with an identical computer. I gave your program a go but sadly it just locked the machine up. Probably way too late to the game, but thought it would be useful feedback. I've posted another topic on the issues I'm experiencing.
Cheers
Andrew

Reply 28 of 29, by Deksor

Posted on 2023-10-09, 08:29

Deksor Offline

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Maybe you have a different bios revision?

Could you please dump it ? 😀

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 29 of 29, by Skip94

Posted on 2023-10-09, 15:12

Skip94 Offline

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Deksor wrote on 2023-10-09, 08:29:

Maybe you have a different bios revision?

Could you please dump it ? 😀

I downloaded the BIOS from TRW, which I belive is your one and it is identical to mine sadly.
Andrew

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