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— blog:pushbx:2022:1111_ldebug_bootunitflx_flag_2_to_force_using_bpb_geometry [2022-11-11 21:11:12 +0100 Nov Fri] (current)
ecm created
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 +====== lDebug: BOOTUNITFLx flag 2 to force using BPB geometry ======
 +
 +After [[blog:pushbx:2022:1111_lcdebug_image_for_use_with_pcjs.org#it_can_be_booted_as_well|having found that some pcjs.org machines report wrong CHS geometries]] on function 13.08, I [[https://hg.pushbx.org/ecm/ldebug/rev/fafa1072a343|prepared a flag]] that can be set in lDebug's ''BOOTUNITFLx'' variables to force falling back to the geometry noted in a unit's BPB. (For now this blindly assumes that the unit starts with a boot sector with BPB, even if the unit is actually >= 80h and partitioned with an MBR.)
 +
 +===== =====
 +
 +This is a hack, and probably will be extended or replaced by something better, but it does work insofar as what it's intended to do. I also prepared a small test case to check that it works, at https://pushbx.org/ecm/test/20221111/geofake.txt
 +
 +<code>0060:0000 B80000            mov     ax, 0000
 +0060:0003 8ED8              mov     ds, ax
 +0060:0005 668B1E4C00        mov     ebx, [004C]
 +0060:000A 0E                push    cs
 +0060:000B 680010            push    word 1000
 +0060:000E 662E891E0020      mov     [cs:2000], ebx
 +0060:0014 668F064C00        pop     dword [004C]
 +0060:0019 90                nop
 +0060:001A CC                int3
 +0060:001B 90                nop
 +
 +0060:1000 80FC08            cmp     ah, 08
 +0060:1003 751B              jnz     1020
 +0060:1005 9C                pushf
 +0060:1006 2EFF1E0020        call    far [cs:2000]
 +0060:100B 7203              jb      1010
 +0060:100D B10F              mov     cl, 0F
 +0060:100F 90                nop
 +0060:1010 55                push    bp
 +0060:1011 89E5              mov     bp, sp
 +0060:1013 D05E06            rcr     byte [bp+06], 1
 +0060:1016 D04606            rol     byte [bp+06], 1
 +0060:1019 5D                pop     bp
 +0060:101A CF                iret
 +
 +0060:1020 2EFF2E0020        jmp     far [cs:2000]</code>
 +
 +Before running the interrupt handler interception, try ''boot dir fda'' (on a typical 1440 KiB 90mm diskette drive). It will list the directory entries. Run the interception. ''boot dir fda'' will show nothing now. Run ''r bootunitfl(0) or= 2''. The directory listing works again.
 +
 +
 +{{tag>ldebug}}
 +
 +
 +~~DISCUSSION~~
  
blog/pushbx/2022/1111_ldebug_bootunitflx_flag_2_to_force_using_bpb_geometry.txt ยท Last modified: 2022-11-11 21:11:12 +0100 Nov Fri by ecm

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