mcb_walk.c: try querying UMB link state even if not on DOS 5+ #161
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Current lMS-DOS still reports its version as v4.00 but does fully support a DOS-managed UMA. As of today lMS-DOS also supports the SHELLHIGH= directive. Like FreeDOS's this is implemented using the 21.4B80 extension. Although the internals differ, the outside observable behaviour is the DOS executable loader allocates both the environment and the process memory block into the UMA, if there are free UMBs. However, unlike LH the UMB link state is not enabled once the control flow is passed to the application. That means an environment block in a UMB isn't reachable when walking the MCBs without regard to proceeding past the first UMCB. The MCB walker is used in FreeCOM's init to validate the environment block passed to it upon its startup. Reference: https://github.com/FDOS/freecom/blob/cede733377c454cb93e9038d476582f0d1fcbf0a/shell/init.c#L238 If this check fails, then the shell will prompt for the position of its executable as follows. This patch makes the MCB walker enable the UMB link even if the reported DOS version is below 5, allowing it to find the environment block's UMCB and avoiding the prompt. Failed to load the strings resource into memory, the location pointed to in %COMSPEC% seems to be invalid. Please specify another location of FreeCOM to try to load the strings from, e.g.: C:\COMMAND.COM or just hit enter to cancel to load the strings.
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In this changeset lMS-DOS adds support for the FreeDOS extension of using the UMA if 21.4B80 is used: https://hg.pushbx.org/ecm/msdos4/rev/1a8906752b66 This is the changeset that adds SHELLHIGH= support: https://hg.pushbx.org/ecm/msdos4/rev/49f66d9e6f0f |
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Linked to this PR from SvarDOS/edrdos#55 (comment) |
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Current lMS-DOS still reports its version as v4.00 but does fully support a DOS-managed UMA. As of today lMS-DOS also supports the SHELLHIGH= directive. Like FreeDOS's this is implemented using the 21.4B80 extension. Although the internals differ, the outside observable behaviour is the DOS executable loader allocates both the environment and the process memory block into the UMA, if there are free UMBs. However, unlike LH the UMB link state is not enabled once the control flow is passed to the application. That means an environment block in a UMB isn't reachable when walking the MCBs without regard to proceeding past the first UMCB.
The MCB walker is used in FreeCOM's init to validate the environment block passed to it upon its startup. Reference:
freecom/shell/init.c
Line 238 in cede733
If this check fails, then the shell will prompt for the position of its executable as follows. This patch makes the MCB walker enable the UMB link even if the reported DOS version is below 5, allowing it to find the environment block's UMCB and avoiding the prompt.