UPI-41A/41AH/42/42AH USER’S MANUAL
PROGRAM MEMORY
The UPI-41A/41AH/42/42AH microcomputer has
1024, 2048 8-bit words of resident, read-only memory
for program storage. Each of these memory locations is
directly addressable by a 10-bit program counter. De-
pending on the type of application and the number of
program changes anticipated, three types of program
memory are available:
#
8041AH, 8042AH with mask programmed ROM
Memory
#
8741AH, 8742AH with electrically programmable
OTP EPROM Memory
#
8741A and 8742 with electrically programmable
EPROM Memory
A program memory map is illustrated in Figure 2-5.
Memory is divided into 256 location ‘pages’ and three
locations are reserved for special use:
231318 – 10
Figure 2-5. Program Memory Map
INTERRUPT VECTORS
1)
Location 0
Following a RESET input to the processor, the next
instruction is automatically fetched from location 0.
2)
Location 3
An interrupt generated by an Input Buffer Full
(IBF) condition (when the IBF interrupt is enabled)
causes the next instruction to be fetched from loca-
tion 3.
3)
Location 7
A timer overflow interrupt (when enabled) will
cause the next instruction to be fetched from loca-
tion 7.
Following a system RESET, program execution begins
at location 0. Instructions in program memory are nor-
mally executed sequentially. Program control can be
transferred out of the main line of code by an input
buffer full (IBF) interrupt or a timer interrupt, or when
a jump or call instruction is encountered. An IBF inter-
rupt (if enabled) will automatically transfer control to
location 3 while a timer interrupt will transfer control
to location 7.
All conditional JUMP instructions and the indirect
JUMP instruction are limited in range to the current
256-location page (that is, they alter PC bits 0 – 7 only).
If a conditional JUMP or indirect JUMP begins in lo-
cation 255 of a page, it must reference a destination on
the following page.
Program memory can be used to store constants as well
as program instructions. The UPI-41AH, 42AH in-
struction set contains an instruction (MOVP3) de-
signed specifically for efficient transfer of look-up table
information from page 3 of memory.
DATA MEMORY
The UPI-41A has 64 8-bit words of Random Access
Memory, the UPI-41AH has 128 8-bit words of Ran-
dom Access Memory; the UPI-42 has 128 8-bit words
of RAM; and the UPI-42AH has 256 8-bit words of
RAM. This memory contains two working register
banks, an 8-level program counter stack and a scratch
pad memory, as shown in Figure 2-6. The amount of
scratch pad memory available is variable depending on
the number of addresses nested in the stack and the
number of working registers being used.
Addressing Data Memory
The first eight locations in RAM are designated as
working registers R
0
– R
7
. These locations (or registers)
can be addressed directly by specifying a register num-
ber in the instruction. Since these locations are easily
addressed, they are generally used to store frequently
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