UPD75238GJ NEC [NEC], UPD75238GJ Datasheet - Page 32

• Manufacturer Part Number: UPD75238GJ
• Description: 4 BIT SINGLE-CHIP MICROCOMPUTER
• Manufacturer: NEC [NEC]

32
3.5 ACCUMULATORS
used for 4-bit data processing instructions, and the XA register pair is mainly used for 8-bit data processing
instructions.
3.6 STACK POINTER (SP) AND STACK BANK SELECT REGISTER (SBS)
address of the stack area is the stack pointer (SP).
banks is selected according to the value of the 4-bit SBS.
(restoration) operation from stack memory. The SBS is set with a 4-bit memory manipulation instruction. Note
that the high-order two bits are always set to 00.
instruction, and the SBS can be initialized with a 4-bit memory manipulation instruction. Both can be read
from as well.
(n: n = 0, 1, 2, or 3) specified with the SBS.
n00H, the operation returns to address nFFH of the same bank. Stacking beyond memory bank boundaries
is enabled only by resetting the SBS.
always be initialized to a desired value at the start of the program.
In the PD75238, the A register and the XA register pair function as accumulators. The A register is mainly
For a bit manipulation instruction, the carry flag (CY) functions as a bit accumulator.
The PD75238 uses static RAM as stack memory (LIFO scheme), and the 8-bit register holding the start
The stack area is located at addresses 000H to 3FFH in memory banks 0, 1, 2, and 3. Either of the memory
The SP is decremented before a write (save) operation to stack memory, and is incremented after a read
Fig. 3-9 and 3-10 show data saved to and restored from stack memory in these stack operations.
To place the stack area at a given location, the SP can be initialized with an 8-bit memory manipulation
When the SP is initialized to 00H, a stack operation starts at the high-order address (nFFH) of memory bank
A stack area must be within the memory bank specified with the SBS. If a stack operation exceeds address
A RESET signal occurrence causes the contents of the SP and the SBS to be undefined, so that the SP must
Table 3-1 Stack Area to Be Selected by the SBS
X
SBS1
0
0
1
1
Fig. 3-7 Accumulators
SBS
SBS2
0
1
0
1
A
A
Memory bank 0
Memory bank 1
Memory bank 2
Memory bank 3
Stack area
CY
Bit accumulator
4-bit accumulator
8-bit accumulator
PD75238