DOS ain't dead - AMIS, Optimize memory usage

AMIS, Optimize memory usage (Developers)

posted by Ninho , 13.05.2011, 11:27

Hullo Japheth

>> That precaution probably seemed necessary to the seasoned (and
>> cautious) professional programmer but,...
>> ... the "good (TM)" programmer (after carefully checking his facts)
> will
>> omit the flag save/restore (thereby reducing the size of the
> > resident KBFR by a whopping 6 bytes iirc) :=)

> Cool!

> This is true ............ to some degree. :))
>
> However ... there are a few considerations:
>
> - int 15h is an interrupt which is used for a lot of purposes.

Indeed the "cassette!" interrupt has been a place holder for an ever expanding number of system services. I consider it the BIOS analogous to DOS's int 2F multiplex ("printer.sys" was it not?)

> - there exists software, including BIOSes, which do NOT return
> from this interrupt by an IRET, but by a RETF 2.

Due to the above, a Retf 2 will make sense in certain cases of int 15h and not in others.

> - a lot of software sets (or resets) the carry flag BEFORE it calls Int
> 15h.

Ack, this is even prescribed in many cases of int 15. Case in point (15/4F) caller (normally an IRQ1 routine) is supposed to SET CY (and I have yet to SEE a firmware BIOS or software IRQ1 handler that doesn't) and our *hooker* returns status information thru the carry, that is the Cy bit of the flag image saved by the initial Int 15 instruction, not the current carry.

> ( this is probably done because there exist buggy hookers/BIOSes ).

Our hooker is not supposed to repair bugs in other software. The important point here is if some other int 15 hooker incorrectly thinks the Flags register passed to it holds meaningful information, there is /no/ general way whatever we do can repair that other software's bug, except in marginal circumstances. I thought this over in the 90s, and I even taught it.

Fortunately when considered in detail the scheme is not as fragile as it looks.

> So, considering these facts(?), one might come to realize that it is at
> least "thinkable" that saving the flags status is not absolutely useless. I
> guess - it was in 1993! - that was the reason why I added it.

This is well understood and why I said the "seasonned, cautious, professional..." would wish to pushf/popf just in case ;=)

Analysis and practice however show that it is in fact both unnecessary, from a functional PoV, and useless as a well meaning measure to "repair" others' bugs. If there was a 1/1,000,000 doubt in my mind, I would gladly put those miserable 6 bytes to the place where I took them off ;=)

Truly,

--
Ninho

---
Ninho

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Complete thread:

int 15/4f French keyboard driver ready + *Ping Japheth* - Ninho, 27.04.2011, 13:34 (Developers)
- int 15/4f French keyboard driver ready + *Ping Japheth* - Japheth, 27.04.2011, 16:02
  - int 15/4f French keyboard driver ready + *Ping Japheth* - Ninho, 27.04.2011, 17:13
    - int 15/4f French keyboard driver ready + *Ping Japheth* - Japheth, 28.04.2011, 07:47
- int 15/4f French keyboard driver ready + Now what? * - Ninho, 10.05.2011, 13:30
  - int 15/4f French keyboard driver ready + Now what? * - Rugxulo, 10.05.2011, 23:52
    - int 15/4f French keyboard driver ready + Now what? * - Ninho, 11.05.2011, 13:38
      - int 15/4f French keyboard driver ready + Now what? * - ecm, 11.05.2011, 19:39
        int 15/4f French keyboard driver ready + Now what? * - Ninho, 12.05.2011, 10:56
        int 15/4f French keyboard driver ready + Now what? * - bretjohn, 12.05.2011, 17:13
        int 15/4f French keyboard driver ready + Now what? * - Ninho, 12.05.2011, 22:27
        AMIS, Optimize memory usage - ecm, 13.05.2011, 00:24
        AMIS, Optimize memory usage - Ninho, 13.05.2011, 01:33
        AMIS, Optimize memory usage - ecm, 13.05.2011, 01:45
        AMIS, Optimize memory usage - Ninho, 13.05.2011, 02:07
        AMIS, Optimize memory usage - ecm, 13.05.2011, 02:39
        AMIS, Optimize memory usage - Ninho, 13.05.2011, 02:58
        Correction, memory usage - Ninho, 13.05.2011, 02:50
        Memory usage, AMIS(!) - ecm, 13.05.2011, 03:00
        Memory usage, AMIS(!) - Ninho, 13.05.2011, 11:57
        Memory usage, AMIS(!) - ecm, 13.05.2011, 14:51
        Memory usage, AMIS(!) - bretjohn, 13.05.2011, 17:43
        Memory usage, AMIS(!) - Ninho, 13.05.2011, 18:15
        Memory usage, AMIS(!) - Japheth, 13.05.2011, 18:58
        Memory usage, AMIS(!) - bretjohn, 14.05.2011, 01:42
        Memory usage, AMIS(!) - Ninho, 14.05.2011, 11:38
        Sorry error :) - No contents - Ignore - Ninho, 14.05.2011, 11:41
        Memory usage, AMIS(!) - bretjohn, 15.05.2011, 17:26
        Int15 handler example: IISP; chain other functions directly - ecm, 15.05.2011, 18:30
        Int15 handler example: IISP; chain other functions directly - Ninho, 16.05.2011, 03:49
        Memory usage, AMIS(!) - Ninho, 15.05.2011, 20:57
        In reply to Bret - int 15/4F stuff - Ninho, 16.05.2011, 03:35
        In reply to Bret - int 15/4F stuff - Japheth, 16.05.2011, 10:18
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - Ninho, 16.05.2011, 12:19
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 16.05.2011, 13:35
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - Ninho, 16.05.2011, 15:10
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 16.05.2011, 17:13
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - bretjohn, 16.05.2011, 19:50
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 16.05.2011, 21:06
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - bretjohn, 17.05.2011, 18:10
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 17.05.2011, 19:47
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - bretjohn, 18.05.2011, 02:09
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - Japheth, 18.05.2011, 06:47
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - bretjohn, 18.05.2011, 19:08
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 18.05.2011, 12:35
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - bretjohn, 18.05.2011, 18:14
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 19.05.2011, 15:32
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - Rugxulo, 19.05.2011, 22:42
        Keyboard, codepages... - Ninho, 19.05.2011, 23:41
        FreeDOS DISPLAY (TSR vs. device driver?) - ecm, 20.05.2011, 14:09
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - Ninho, 19.05.2011, 23:59
        In reply to : Bret 'll have to hate FreeDOS mKEYB, too - tom, 20.05.2011, 00:49
        In reply to Bret... - Ninho, 18.05.2011, 15:41
        DOS IO kernel part hooking Int15.4F ? - ecm, 18.05.2011, 20:35
        DOS IO kernel part hooking Int15.4F ? - Ninho, 18.05.2011, 22:41
        DOS kernel, Int15.4F - ecm, 18.05.2011, 22:48
        In reply to Bret... - tom, 19.05.2011, 15:15
        In reply to \\\ Tom... - Ninho, 19.05.2011, 19:27
        AMIS, Optimize memory usage - Japheth, 13.05.2011, 07:28
        AMIS, Optimize memory usage - Ninho, 13.05.2011, 11:27
        AMIS, No flaming - ecm, 13.05.2011, 00:24
        AMIS - ecm, 13.05.2011, 00:24
        AMIS - Ninho, 13.05.2011, 02:21
        AMIS, Ciriaco - ecm, 13.05.2011, 03:23
        AMIS, Ciriaco, Ralph ? - Ninho, 13.05.2011, 12:16
      - int 15/4f French keyboard driver ready + Now what? * - Rugxulo, 11.05.2011, 20:12
        int 15/4f French keyboard driver ready + Now what? * - Ninho, 12.05.2011, 10:36