Xerox Alto I User Manual

Alto Hardware Manual
Section 7: Ethernet
55
* These functions have a peculiar timing restriction associated with them. The microinstruction that
executes one
of them must stop the clock for one cycle. On Alto I, the microprogrammer must do
this
using memory timing (i.e., by referencing MD in the same microinstruction, during the third or
fourth cycle of a memory reference). On Alto II, the hardware automatically stops the clock for
one cycle when necessary; however, due
to a design error, the instruction following the one
specifying
EIDFCf or EILFCT is occasionally stopped instead. Consequently, the programmer must
not permit a task switch to occur between these
two microinstructions, nor start a memory
reference in the following microinstruction.

Xerox Alto I Specifications

General

Category	Desktop
Manufacturer	Xerox
Model	Alto I
Release Year	1973
Operating System	Alto Executive
Pointing Device	Mouse
Input Devices	Keyboard, Mouse
Networking	Ethernet
Memory	128 KB (expandable to 512 KB)
Storage	2.5 MB removable cartridge
Display	606x808 monochrome