RCA COSMAC Operator's Manual page 65

Sample Program System (cont'd)
A simple change in the Sample Program code would
permit an even simpler interface card -- one requiring
no IC chips at all! This illustrates the concept of
"hardware/software trade-offs" possihle in micro-
computer systems. The Sample Program reads in eight
bytes by executing an I/O Read whenever it sees EFI
true.
Suppose it waited for EFI to be false before
it looked for EFI true. All of the switch debounce
hardware can now be eliminated. The program becomes
longer by two bytes as follows:
Old EFI test (address OOOA): 3COA
New EFI test (address OOOA):
330A3COC
The new code causes the program to loop at address OOOA
until EFI becomes false, then it loops at OOOC until it
becomes true again*. The switch logic can be replaced by:
ENTER
Ir------e>
,CJ
lTj
Recall that EFI-N is pUlled
I
T
C
TiT
up by
22K
EFI-N
on the CPU module.
*
If you should want to try this program, remember that
all addresses'below OOOC should be increased by 2.
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