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C EN TRAL PROC ESSIN G UN IT
2.4 Program Counter (PC)
The Program Counter is a sixteen-bit counter consisting of PC
eight-bit registers. The contetnts of the Program Counter indicates the address which an
instruction to be executed next is stored.
The 740 Family uses a stored program system; to start a new operation it is necessary to
transfer the instruction and relevant data from memory to the CPU.
Normally the Program Counter is used to indicate the next memory address. After each
instruction is executed, the next instruction required is read. This cycle is repeated until the
program is finished.
The control of the Program Counter of the 740 Family is almost fully automatic. However,
caution must be exercised to avoid differences between program flow and Program
Counter contents when using the Stack Pointer or directly altering the contents of the
Program Counter.
2.5 Processor Status Register (PS)
The Processor Status Register is an eight-bit register consisting of 5 flags which indicate the
status of arithmetic operations and 3 flags which determine operation.
Each of these flags is described below. Table 2.5.1 lists the instructions to set/clear each flag.
Refer to the section "Appendix 2 MACHINE LANGUAGE INSTRUCTION TABLE" or "3.3
INSTRUCTIONS" for details on when these flags are altered.
[ Carry flag C ] ------------------------------------------------------ Bit 0
This flag stores any carry or borrow from the Arithmetic Logic Unit (ALU) after an arithmetic
operation and is also changed by the Shift or Rotate instruction.
This flag is set by the SEC instruction and is cleared by the CLC instruction.
[ Zero flag Z ] ------------------------------------------------------- Bit 1
This flag is set when the result of an arithmetic operation or data transfer is "0" and is
cleared by any other result.
[ Interrupt disable flag I ] ---------------------------------------- Bit 2
This flag disables interrupts when it is set to "1." This flag immediately becomes "1" when
an interrupt is received.
This flag is set by the SEI instruction and is cleared by the CLI instruction.
[ Decimal mode flag D ] ----------------------------------------- Bit 3
This flag determines whether addition and subtraction are performed in binary or decimal
notation. Addition and subtraction are performed in binary notation when this flag is set to
"0" and as a 2-digit, 1-word decimal numeral when set to "1." Decimal notation correction
is performed automatically at this time.
This flag is set by the SED instruction and is cleared by the CLD instruction.
Only the ADC and SBC instructions are used for decimal arithmetic operations.
Note that the flags N, V and Z are invalid when decimal arithmetic operations are per-
formed by these instructions.
[ Break flag B ] ----------------------------------------------------- Bit 4
This flag determines whether an interrupt was generated with the BRK instruction. When a
BRK instruction interrupt occurs, the flag B is set to "1" and saved to the stack; for all other
interrupts the flag is set to "0" and saved to the stack.
4
Program Counter (PC)
Processor Status Register (PS)
and PC
H
L
, which are each
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