DOS ain't dead

I added the basics for HMA support to the MS-DOS based lDOS. Only used for the msdos-based DOSCODE segment or explicit users of the 2F.4A interface yet. During this I also had to add support for working with A20, enabling it when entering the DOS and disabling on exec branching into a new process. (21.4B05 support not yet added.) I found that the FreeDOS kernel uses functions 05h and 06h to enable and disable A20: https://github.com/FDOS/kernel/blob/5de2eb1aa01b0129b046cf06338453a233c6597b/kernel/kernel.asm#L1235 These functions are called Local Enable A20 and Local Disable A20: https://fd.lod.bz/rbil/interrup/memory/2f4310.html#table-02753 Previously I used the 03h and 04h Global Enable A20 and Global Disable A20. But I found that these didn't work as intended sometimes (cannot reproduce it right now). So I first changed to 05h and 06h, but at least once my test program showed A20 on initially the first time I ran it, where I expected A20 off. Calling both global disable and local disable fixed this. I'm using (so far, only) 03h Global Enable A20 in init.asm to relocate DOSCODE: https://hg.pushbx.org/ecm/msdos4/file/ff139b58d0cd/src/BIOS/init.asm#l3661 In entry.asm I'm using 05h Local Enable on entry into DOS now, but both Local and Global Disable: https://hg.pushbx.org/ecm/msdos4/rev/ff139b58d0cd What is the exact meaning of "Local" and "Global"? Here's a test program I created, it tries to disable A20 (either with XMS or int 15h) and reports the status 4 times: https://pushbx.org/ecm/test/20250131/a20test.asm It also uses both Global and Local Disable for now. ; Public Domain         cpu 8086         org 256 start:         int3 before:         call test         mov dx, msg.offbefore         je .off .on:         mov dx, msg.onbefore .off:         mov ah, 09h         int 21h after:         call test         mov dx, msg.offafter         je .off .on:         mov dx, msg.onafter .off:         mov ah, 09h         int 21h switchoff:         mov ax, 4300h         int 2Fh         cmp al, 80h         jne noxms         mov dx, msg.tryingxms         mov ah, 09h         int 21h         mov ax, 4310h         int 2Fh         push es         push bx         mov bp, sp         mov ah, 04h         call far [bp]         mov ah, 06h         call far [bp]         jmp end noxms:         mov dx, msg.tryingrombios         mov ah, 09h         int 21h         mov ax, 2400h         int 15h end: testfirst:         call test         mov dx, msg.off         je .off .on:         mov dx, msg.on .off:         mov ah, 09h         int 21h testagain:         call test         mov dx, msg.offagain         je .off .on:         mov dx, msg.onagain .off:         mov ah, 09h         int 21h         mov ax, 4C00h         int 21h test:         les di, [cs:high]         lds si, [cs:low]         mov cx, 16         repe cmpsw         push cs         pop ds         retn align 2 high:   dd 0FFFF_0010h low:    dd 0 msg: .offbefore: .off:   db "A20 was off. ",36 .onbefore: .on:    db "A20 was on. ",36 .offafter: .offagain:      db "After int 21h A20 is off.",13,10,36 .onafter: .onagain:       db "After int 21h A20 is on.",13,10,36 .tryingxms:     db "XMS detected, trying to disable A20 on XMS.",13,10,36 .tryingrombios: db "XMS not detected, trying to disable A20 using ROM-BIOS.",13,10,36

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The specifications says: https://www.phatcode.net/res/219/files/xms20.txt Global Enable A20 (Function 03h): --------------------------------- ARGS: AH = 03h RETS: AX = 0001h if the A20 line is enabled, 0000h otherwise ERRS: BL = 80h if the function is not implemented BL = 81h if a VDISK device is detected BL = 82h if an A20 error occurs This function attempts to enable the A20 line. It should only be used by programs which have control of the HMA. The A20 line should be turned off via Function 04h (Global Disable A20) before a program releases control of the system. NOTE: On many machines, toggling the A20 line is a relatively slow operation. Global Disable A20 (Function 04h): ---------------------------------- ARGS: AH = 04h RETS: AX = 0001h if the A20 line is disabled, 0000h otherwise ERRS: BL = 80h if the function is not implemented BL = 81h if a VDISK device is detected BL = 82h if an A20 error occurs BL = 94h if the A20 line is still enabled This function attempts to disable the A20 line. It should only be used by programs which have control of the HMA. The A20 line should be disabled before a program releases control of the system. NOTE: On many machines, toggling the A20 line is a relatively slow operation. Local Enable A20 (Function 05h): -------------------------------- ARGS: AH = 05h RETS: AX = 0001h if the A20 line is enabled, 0000h otherwise ERRS: BL = 80h if the function is not implemented BL = 81h if a VDISK device is detected BL = 82h if an A20 error occurs This function attempts to enable the A20 line. It should only be used by programs which need direct access to extended memory. Programs which use this function should call Function 06h (Local Disable A20) before releasing control of the system. NOTE: On many machines, toggling the A20 line is a relatively slow operation. Local Disable A20 (Function 06h): --------------------------------- ARGS: AH = 06h RETS: AX = 0001h if the function succeeds, 0000h otherwise ERRS: BL = 80h if the function is not implemented BL = 81h if a VDISK device is detected BL = 82h if an A20 error occurs BL = 94h if the A20 line is still enabled This function cancels a previous call to Function 05h (Local Enable A20). It should only be used by programs which need direct access to extended memory. Previous calls to Function 05h must be canceled before releasing control of the system. NOTE: On many machines, toggling the A20 line is a relatively slow operation.

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From memory, so be careful: Global enable/disable unconditionally sets the A20 line Local enable/disable increments/decrements a counter. only if the counter switches between 0 and !=0 the A20 line is changed. So if LocalEnable() is called 10 times, LocalDisable() must also be called 10 times before the actual A20 status changes back to disabled. However, I may be totally wrong Fortunately, the source to MS HIMEM is (and always was) publicly available, and can be regarded as the definition of these functions. FD HIMEM is supposed to mimic it precisely (which is not certain). So you can look it up yourself without relying on unprecise descriptions. [Edited to add]However I am not aware of a guideline how these functions are to be used, for what purpose. If someone knows, please report.