Public bug reported:
If you run QBasic in qemu, printing a double-type single-digit number
will print an extra decimal point (e.g. PRINT CDBL(3) prints "3.") that
does not appear when running on a real CPU (or on qemu with -enable-
kvm). I tracked this down to the state of the status flags after a
loopnz instruction.
After executing a sequence like this in qemu:
mov bx,1
mov cx,1
dec bx ; sets Z bit in flags
A: loopnz A ; should not modify flags
Z is incorrectly clear afterwards. loopz does the same thing (but not
plain loop). Interestingly, inserting pushf+popf after dec results in Z
set, so loopnz/loopz does not always clear Z itself but is rather
interfering with the previous instruction's flag setting.
Version 5.1.0-rc0, x86-64 host.
** Affects: qemu
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1888165
Title:
loopz/loopnz clearing previous instruction's modified flags on cx -> 0
Status in QEMU:
New
Bug description:
If you run QBasic in qemu, printing a double-type single-digit number
will print an extra decimal point (e.g. PRINT CDBL(3) prints "3.")
that does not appear when running on a real CPU (or on qemu with
-enable-kvm). I tracked this down to the state of the status flags
after a loopnz instruction.
After executing a sequence like this in qemu:
mov bx,1
mov cx,1
dec bx ; sets Z bit in flags
A: loopnz A ; should not modify flags
Z is incorrectly clear afterwards. loopz does the same thing (but not
plain loop). Interestingly, inserting pushf+popf after dec results in
Z set, so loopnz/loopz does not always clear Z itself but is rather
interfering with the previous instruction's flag setting.
Version 5.1.0-rc0, x86-64 host.
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