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Stack Pointer
Stack Pointer
Stack is a LIFO( Last in, First out) structure implemented in the RAM area and is used to store addresses and data, when the microprocessor branches to a subroutine.
Stack is a LIFO( Last in, First out) structure implemented in the RAM area and is used to store addresses and data, when the microprocessor branches to a subroutine.
These are two special purpose registers each of 16 bit width.
These are two special purpose registers each of 16 bit width.
1. Stack Pointer(SP)
1. Stack Pointer(SP)
2. Program Counter (PC)
2. Program Counter (PC)
SP is special purpose 16 bit register. It contains a memory address. Suppose SP contents are FC78H, then the 8085 interprets it as follows.
SP is special purpose 16 bit register. It contains a memory address. Suppose SP contents are FC78H, then the 8085 interprets it as follows.
Memory locations FC78H, FC79H…..FFFFH are having useful information. In other words these locations are treated as filled locations.
Memory locations FC78H, FC79H…..FFFFH are having useful information. In other words these locations are treated as filled locations.
Memory locations FC77H, FC76H….0000H are not having any useful information. These locations are treated as empty.
Memory locations FC77H, FC76H….0000H are not having any useful information. These locations are treated as empty.
Thus the contents of SP specify the top most useful location in the stack. In other words it indicate memory location with the smallest address having useful information.
Thus the contents of SP specify the top most useful location in the stack. In other words it indicate memory location with the smallest address having useful information.
Reading from the STACK
Reading from the STACK
Right now , memory location FC78H,
Right now , memory location FC78H,
FC79H----FFFFH are all interpreted by 8085 to have useful information.
FC79H----FFFFH are all interpreted by 8085 to have useful information.
Then which one of useful information to read, 8085 reads from the top of the stack of useful information.
Then which one of useful information to read, 8085 reads from the top of the stack of useful information.
8085 is designed to read always 2-bytes useful information from the top of stack.
8085 is designed to read always 2-bytes useful information from the top of stack.
As such in this case it reads from location FC78H and FC79.
As such in this case it reads from location FC78H and FC79.
In 8085 this information which is read can only be loaded into a register pair.
In 8085 this information which is read can only be loaded into a register pair.
This operation of loading register pair by reading information from the stack top is called a POP operation.
This operation of loading register pair by reading information from the stack top is called a POP operation.
Suppose we load register BC when SP contents was FC78H. Then information from memory locations FC78H and FC79H are copied to BC pair. But after this pop operation, the contents of FC78H and FC79H are treated as useless by 8085. This is because a copy is there in the register pair BC anyway!
Suppose we load register BC when SP contents was FC78H. Then information from memory locations FC78H and FC79H are copied to BC pair. But after this pop operation, the contents of FC78H and FC79H are treated as useless by 8085. This is because a copy is there in the register pair BC anyway!
Thus only FC7Ah,FC7BH,-----,FFFFH are useful locations.
Thus only FC7Ah,FC7BH,-----,FFFFH are useful locations.
To indicate this , the SP contents are changed to FC7AH.
To indicate this , the SP contents are changed to FC7AH.
This is done automatically by 8085 by incrementing SP contents by 2.
This is done automatically by 8085 by incrementing SP contents by 2.
Writing to the Stack
Writing to the Stack
Let us say SP contents are FC7AH and we want to write information to a stack location. In this case we are not interested in writing to a location whose address is equal or greater than the memory address present in SP. This is because the 8085 interprets them as having useful information which should not be destroyed.
Let us say SP contents are FC7AH and we want to write information to a stack location. In this case we are not interested in writing to a location whose address is equal or greater than the memory address present in SP. This is because the 8085 interprets them as having useful information which should not be destroyed.
For example, there is no point in overwriting and destroying useful information at memory locationFD7AH. We should be writing into a location where there is presently useless information and make it useful.
For example, there is no point in overwriting and destroying useful information at memory locationFD7AH. We should be writing into a location where there is presently useless information and make it useful.
Right now, memory locations FC79H,FC78H,-----,0000H are all interpreted by 8085 to have useless information. Then which one of these locations to over write?
Right now, memory locations FC79H,FC78H,-----,0000H are all interpreted by 8085 to have useless information. Then which one of these locations to over write?
The 8085 is designed to write always into two locations of useful information just above the top of stack. As such in this case it writes to location of useless information just above the top of stack.
The 8085 is designed to write always into two locations of useful information just above the top of stack. As such in this case it writes to location of useless information just above the top of stack.
In this case it writes to locations FC79H and FC78H.
In this case it writes to locations FC79H and FC78H.
In 8085 this information which is written can only be coming from register pair.
In 8085 this information which is written can only be coming from register pair.
This operation of storing a register pair by writing information above the stack top is called PUSH operation.
This operation of storing a register pair by writing information above the stack top is called PUSH operation.
Suppose we store register pair BC when SP contents were FC7A H then information from BC pair is stored in memory location FC79H and FC78H. But after this PUSH operation the contents of FC79H and FC78H are treated as useful by the 8085. This is because nobody stores useless information. Tus FC78H,FC79H,----,FFFFH are all useful locations.
Suppose we store register pair BC when SP contents were FC7A H then information from BC pair is stored in memory location FC79H and FC78H. But after this PUSH operation the contents of FC79H and FC78H are treated as useful by the 8085. This is because nobody stores useless information. Tus FC78H,FC79H,----,FFFFH are all useful locations.
To indicate this the SP contents are changed to FC78H.
To indicate this the SP contents are changed to FC78H.
This is done automatically by 8085 by decrementing SP contents by 2.
This is done automatically by 8085 by decrementing SP contents by 2.
Instruction type POP rp
Instruction type POP rp
This instruction loads register pair rp by popping out 2-bytes from the top of the stack.
This instruction loads register pair rp by popping out 2-bytes from the top of the stack.
In previous chapters, ‘rp’ stood for any of the register pairs BC, DE, or HL.
In previous chapters, ‘rp’ stood for any of the register pairs BC, DE, or HL.
We can also treat combination of accumulator and flags as one more register pair called PSW stands for Processor Status word.
We can also treat combination of accumulator and flags as one more register pair called PSW stands for Processor Status word.
Accumulator is MS byte and flag register is the LS byte.
Accumulator is MS byte and flag register is the LS byte.
Two bits are used in an opcode to specify a register pair. Thus actually four register pair can be specified using 2-bit code for rp. In fact even SP can be treated as a register pair.
Two bits are used in an opcode to specify a register pair. Thus actually four register pair can be specified using 2-bit code for rp. In fact even SP can be treated as a register pair.
Thus in all five register pairs.
Thus in all five register pairs.
One may think to specify all register pairs we may need 3 bits. But 8085 opcodes use only 2 bits to specify a register pair as shown below. The rp code of 11 specify either SP or PSW, but not for both.
One may think to specify all register pairs we may need 3 bits. But 8085 opcodes use only 2 bits to specify a register pair as shown below. The rp code of 11 specify either SP or PSW, but not for both.
For example POP rp instruction can be BC, DE, HL or PSW. There is no instruction like POP SP.
For example POP rp instruction can be BC, DE, HL or PSW. There is no instruction like POP SP.
Similarly, in LXI rp instruction rp can be BC DE HL or SP.
Similarly, in LXI rp instruction rp can be BC DE HL or SP.
There is no instruction like LXI PSW.
There is no instruction like LXI PSW.
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