1. Part of the Assembler Language Programmers Toolbox
Register types
Part of the Assembler Language Programmers Toolbox
Copyright © Curt Hill

2. Copyright © 2005- 2007 - Curt Hill
Registers
Definition: A group of bits of storage in the CPU
Two kinds
Special purpose
General purpose
The special purpose are fading in popularity as the hardware designers get more sophisticated
What is actually happening is that a general purpose register is a front for several special purpose registers
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The location
Somewhat insignificant
They may be logically in CU or ALU
Often accessible by both
Sometimes a communication mechanism between the two
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Instruction register
A location where the instruction is stored for decoding
The results determine the rest of the execution of the instruction
This one we can usually disregard at this level because it is not programmer accessible on any machine that I am aware of
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Program counter
Address of the next instruction
A branch is essentially a load of the the program counter
The value in the PC is incremented by the size of the instruction (which may be variable)
AKA instruction pointer
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Status register
Has bits that indicate the operation of the program
One or two bits indicate the result of a comparison or arithmetic:
Greater or positive
Zero or equal
Less or negative
Overflow
Carry out
Parity
Whether the CPU is in supervisor mode or not
Protection key of memory
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Example PSW of IBM 370
This combined the PC and Status flags
Two full words 64 bits
Channel mask 0-6
External mask 7
Masks 12-15
14 wait
13 machine check
12 Basic control (360 mode)
Interruptions code 16-31
Instruct Length Code 32-33
Condition code (34-35)
Program mask (36-39)
Instruction address (40-63)
It had a different interpretation in Extended Control Mode than Basic
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Accumulator
The site of arithmetic and logical instructions
AKA a DATA register
Usually a relatively primitive register in less developed CPUs
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Address register
Used to access data or instructions in the program
Some examples:
Base registers
Segment registers
Index registers
The full discussion of address registers will have to wait for our discussion of addressing modes
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Base register
A register specified in an instruction
Generally added to an offset that is also in the instruction
The contents of the register and offset are summed to provide an effective address
The number of bits of offset determine the block addressed by the register
System 360 had 12 bits
Each block was 4K
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Segment register
Used when memory is physically or logically divided into segments
Each segment is usually a power of two long
The 86 code register addressed 64K
A real address then becomes the contents of the register plus some offset built into the instruction
This may also include a base or index register
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Index register
Usually a base register addresses a block of variables
An index register usually hold a pre-multiplied array subscript
It is usually used in addition to an offset and base or segment register
Thus a memory reference may include the sum of three registers and an offset
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Stack pointer
Special type that allows pushes and pops without much regard to length
Often allows a displacement to be added for indirection
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14. General Purpose Registers
General registers are able to do multiple things
Accumulator or Data
Base or index
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Floating point
Floating point registers are usually separate from the integer portions
The integer registers are used for almost every instruction
By comparison floating point is much less frequently used
Many computers had no floating point at all or it was optional
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IBM 360 – Model 30 and above
16 general purpose registers
Each is 32 bits
All are general purpose with a few exceptions:
R0 always supplies a zero when used as an index register
Some instructions like the Multiply require a even odd pair of registers
There is also the before mentioned PSW, which is not really user accessible
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Intel 8086
General registers: AX BX CX DX
Pointer and index
Stack SP
Base BP
Source Index SI
Destination DI
Segment
Code
Data
Stack
Extra
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Intel 80386
When they went 32 bit they had to double the register size
Therefore they have EAX which is the entire 32 and AX which is the lower 16 for compatibility with old 86
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Motorola 68000
8 data registers
8 address registers
The last of which is stack pointer
Two versions of this: one for system and one for user
Status register
Status and program counter
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