The 8088 (maybe 8086 too) had a bug where an interrupt during a “rep movsb” would return to the rep, so any prefixes were lost. You had to do e.g. “rep cs: mov

The 8088 (maybe 8086 too) had a bug where an interrupt during a “rep movsb” would return to the rep, so any prefixes were lost. You had to do e.g. “rep cs: movsb” instead of “cs: rep movsb” because the latter would switch to ds: on return from interrupt. Fun times.