Stupid 80 column mode question

[Ross Archer <archer@topnow.com>]

I have an Apple IIe, with the 80 column card and 64K of extra RAM.
While I've gleaned from the Apple Tech Reference manual (for the IIc,
not IIe) which locations are involved in setting 80 column mode, when
I try to do this I get half-a-screen of 80 column and garbage on the
other half.

Is there a reliable way to switch to 80 column text, such that I can
use ProDOS, BASIC, and the assembler/monitor in 80 column mode?

Odd that I don't know how to do this, but my only exposure to Apple ][s
was my friend's Apple ][+, which was way pre-80 column mode era.

Also, anyone know where I can find a copy of Merlin Assembler?

Thanks for any info.

-- Ross