Holtek HT46R47 - Stack; Arithmetic and Logic Unit

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Stack

This is a special part of the memory which is used to save the contents of the Program Counter

only. The stack can have between 6, 8 or 16 levels depending upon which device is selected and is

neither part of the data nor part of the program space, and is neither readable nor writable. The acti

vated level is indexed by the stack pointer (SP) and is neither readable nor writeable. At a subrou

tine call or interrupt acknowledge signal, the contents of the Program Counter are pushed onto the

stack. At the end of a subroutine or an interrupt routine, signaled by a return instruction (RET or

RETI), the Program Counter is restored to its previous value from the stack. After a chip reset, the

SP will point to the top of the stack.

If the stack is full and an enabled interrupt takes place, the interrupt request flag will be recorded

but the acknowledge signal will be inhibited. When the stack pointer is decremented (by RET or

RETI), the interrupt will be serviced. This feature prevents stack overflow allowing the program

mer to use the structure more easily. However, when the stack is full, a CALL subroutine instruc

tion can still be executed which will result in a stack overflow. Precautions should be taken to avoid

such cases which might cause unpredictable program branching.

Note

1. For the HT46R47/HT46C47 and HT46R22/HT46C22, N=6, i.e. 6 levels of stack available.

2. For the HT46R23/HT46C23, N=8, i.e. 8 levels of stack available.

3. For the HT46R24/HT46C24, N=16, i.e. 16 levels of stack available.

Arithmetic and Logic Unit - ALU

The arithmetic-logic unit or ALU is a critical area of the microcontroller that carries out arithmetic

and logic operations of the instruction set. Connected to the main microcontroller data bus, the

ALU receives related instruction codes and performs the required arithmetic or logical operations

after which the result will be placed in the specified register. As these ALU calculation or opera

tions may result in carry, borrow or other status changes, the status register will be correspond

ingly updated to reflect these changes. The ALU supports the following functions:

Arithmetic operations ADD, ADDM, ADC, ADCM, SUB, SUBM, SBC, SBCM, DAA

Logic operations AND, OR, XOR, ANDM, ORM, XORM, CPL, CPLA

Rotation RRA, RR, RRCA, RRC, RLA, RL, RLCA, RLC

Increment and Decrement INCA, INC, DECA, DEC

Branch decision, JMP, SZ, SZA, SNZ, SIZ, SDZ, SIZA, SDZA, CALL, RET, RETI

A/D Type MCU

P r o g r a m C o u n t e r

S t a c k L e v e l 1

S t a c k L e v e l 2

S t a c k L e v e l 3

S t a c k L e v e l N

P r o g r a m

M e m o r y

T o p o f S T A C K

S t a c k

P o i n t e r

B o t t o m o f S T A C K

Holtek HT46R47 - Stack; Arithmetic and Logic Unit - ALU