LH share.com crashes older dosemu2

[ecm-pushbx]

I just rebuilt gcc ia16 (with some test failures) and compiled SHARE with that. Using (FreeCOM) LH makes share.com crash the machine. This is true even if SHARE is already installed, so the crash appears to occur early, before actually installing the resident program.

This is on dosemu2 Build 2.0pre8-20190827-1234-g0984731c9 and using the following FreeCOM and FreeDOS kernel:

FreeCom version 0.85a - WATCOMC - XMS_Swap [Jul 10 2021 19:28:06]
DOS version 7.10
FreeDOS kernel - SVN (build 2042 OEM:0xfd) [compiled Jan 24 2020]

Using the same SHARE, kernel, and shell on Build 2.0pre8-20210410-2880-g65853ec95 results in no crash, LH for SHARE works as expected.

(I would report this to dosemu2 but the newer revision appears to work as expected.)

@andrewbird do you have any idea what this may be about? I uploaded my build of share.com with https://github.com/FDOS/share/pull/6

[ecm-pushbx]

The error appears to be in the first relocation early on. Working in newer dosemu2:

Welcome to dosemu2!
    Build 2.0pre8-20210410-2880-g65853ec95
E:\>lh ldebug share.com
-r
AX=0000 BX=0000 CX=0F5A DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
DS=C309 ES=C309 SS=C309 CS=C309 IP=0100 NV UP EI PL ZR NA PE NC
C309:0100 E9150A            jmp     0B18
-t
AX=0000 BX=0000 CX=0F5A DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B18 NV UP EI PL ZR NA PE NC
C309:0B18 BE5810            mov     si, 1058
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=FFFE BP=0000 SI=1058 DI=0000
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B1B NV UP EI PL ZR NA PE NC
C309:0B1B BFDEFE            mov     di, FEDE
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=FFFE BP=0000 SI=1058 DI=FEDE
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B1E NV UP EI PL ZR NA PE NC
C309:0B1E B99602            mov     cx, 0296
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=FFFE BP=0000 SI=1058 DI=FEDE
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B21 NV UP EI PL ZR NA PE NC
C309:0B21 FD                std
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=FFFE BP=0000 SI=1058 DI=FEDE
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B22 NV DN EI PL ZR NA PE NC
C309:0B22 F3A5              rep     movsw
-tp
AX=0000 BX=0000 CX=0000 DX=0000 SP=FFFE BP=0000 SI=0B2C DI=F9B2
DS=C309 ES=C309 SS=C309 CS=C309 IP=0B24 NV DN EI PL ZR NA PE NC
C309:0B24 B430              mov     ah, 30
-

Same run in the older dosemu crashes on the TP step (proceed past the repeated string operation):

Welcome to dosemu2!
    Build 2.0pre8-20190827-1234-g0984731c9
G:\>lh ldebug share.com
-r
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=0000 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0100 NV UP EI PL ZR NA PE NC
F068:0100 E9150A            jmp     0B18
-t
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=0000 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B18 NV UP EI PL ZR NA PE NC
F068:0B18 BE5810            mov     si, 1058
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=1058 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B1B NV UP EI PL ZR NA PE NC
F068:0B1B BFDEFE            mov     di, FEDE
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B1E NV UP EI PL ZR NA PE NC
F068:0B1E B99602            mov     cx, 0296
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B21 NV UP EI PL ZR NA PE NC
F068:0B21 FD                std
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B22 NV DN EI PL ZR NA PE NC
F068:0B22 F3A5              rep     movsw
-tpERROR: unexpected CPU exception 0x06 err=0x00000000 cr2=bf1d273c while in vm86 (
DOS)
Real-mode state dump:
EIP: ffff:000015b5 ESP: 0361:0000b93c  VFLAGS(b): 00011 00110000 10010010
EAX: 0000ffd0 EBX: 0000a250 ECX: 0000d606 EDX: 000036c9 VFLAGS(h): 00033092
ESI: 00000361 EDI: 00000361 EBP: 00004000 DS: 0361 ES: 0361 FS: 0000 GS: 0000
FLAGS: AF SF IF RF VM  IOPL: 3
STACK: ca 36 00 40 61 03 61 03 b5 15 -> 50 a2 f7 0e 46 32 61 03 a4 a4
OPS  : 0e 9e e9 1f fe e9 1c fe e9 19 -> fe b8 0c 81 5f 5e c9 cb c7 13
        fe                  ffff:15b5 db FE

Terminated with signal 4

Does anyone spot anything here?

[ecm-pushbx]

The address is different. Let me check whether the memory block is large enough.

[ecm-pushbx]

Got it! The different UMB layout makes it so SHARE is loaded into a block smaller than 64 KiB. Here's the error:

Welcome to dosemu2!
    Build 2.0pre8-20190827-1234-g0984731c9
G:\>lh ldebug share.com
-dm
PSP: F068
02B4 4D 0008 0019    400 B SD
02CE 4D 02CF 0091    2 KiB COMMAND
0360 4D 0361 1B34  108 KiB LDEBUG
1E95 4D 0000 8169  517 KiB
9FFF 4D 0008 3100  196 KiB SC
D100 4D 0008 1EFF  123 KiB SC
F000 4D 02CF 0040   1024 B COMMAND
F041 4D 0361 0012    288 B LDEBUG
F054 4D F068 0012    288 B SHARE
F067 5A F068 0B98   46 KiB SHARE
-r
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=0000 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0100 NV UP EI PL ZR NA PE NC
F068:0100 E9150A            jmp     0B18
-t
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=0000 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B18 NV UP EI PL ZR NA PE NC
F068:0B18 BE5810            mov     si, 1058
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=1058 DI=0000
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B1B NV UP EI PL ZR NA PE NC
F068:0B1B BFDEFE            mov     di, FEDE
-
AX=0000 BX=0000 CX=0F5A DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B1E NV UP EI PL ZR NA PE NC
F068:0B1E B99602            mov     cx, 0296
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B21 NV UP EI PL ZR NA PE NC
F068:0B21 FD                std
-
AX=0000 BX=0000 CX=0296 DX=0000 SP=B97E BP=0000 SI=1058 DI=FEDE
DS=F068 ES=F068 SS=F068 CS=F068 IP=0B22 NV DN EI PL ZR NA PE NC
F068:0B22 F3A5              rep     movsw
-h linear es:di+2
00100560  decimal: 1049952
-h linear f068+b98:0
000FC000  decimal: 1032192
-

The linear address behind the last-written word is in the HMA (at 10_0560h), extending fairly far behind the end of SHARE's MCB (at F_C000h). I will report this for the compiler, it really shouldn't assume it has 64 KiB available for a flat .COM style executable.