Action Types

Another view of ISA is what is its function. This can be broken into 
several catagories. Various commands in these catagories may overlap, 
but they are listed by their primary function.

Program control - Jump, Call, Interrupt and the appropriate returns. 
These may be an absolute value or relative to the current IP settings.  
In general, each of these behaved slightly different.

<ul>
  Jump - set the IP to a new location where the program continued. There 
  is no return data stored or other changes. Available on all systems.       
  <p>

  Call - refered to by several different names. The Call records the 
  address of the command immediately following the Call and a matching 
  command retrieves this information to return to the calling rountine. 
  This can be implemented in a variety of ways that can take a large or 
  small amount of system resources. On Intel, the return data is stored 
  on the stack which has its own pointer and storage area. On the IBM 
  360, the user specifys the register that will hold the return address 
  until needed and its the user's responsibility to not destroy that 
  information.
  <p>

  Interrupt - An interrupt usually records the return address, and the 
  conditon status of the current process environment. On some systems, 
  it may record even more. Some systems allow masking or restriction of 
  different interrupt requests or request types.
  <p>

  Conditionals - Most systems have conditional versions of the above
  command types.  Often address calculations switch to relative addressing.
  <p>

  Return - Both the calls and interrupts require matching return commands.  
  These implicit instructions invoke standardized actions whose outcome is
  based on the previous state of the system
  <p>

</ul>
Data movement - movement between the registers and memory, or register and
register.

Data manipulation - changing the data in some way.

<ul>
  Boolean - AND, OR, XOR, NOT performed at the byte or register size level.
  <p>

  Single operand manipulation instructions - negate, either 1s or 2s
  complement, increment (usually by 1), decrement, and zero. On some
  systems, multiple actions may be included in the command, such as
  decrement and branch if counter = 0.
  <p> 

  Bit Manipulation  - Set, Clear and test using masks. Offers several 
  of the boolean and single op commands applied to individual bits 
  rather than to the whole byte or word.
  <p>

  Shift and Rotate - shifting the data unit in left or right. This can 
  be used to effectively multiply or divide. Additionally, it can be 
  used to simulate some of the bit manipulation commands. Shifts can 
  vary in their behavior. Bits shifted out may be lost or preserved and 
  bits shifted in may be zero or obtained from somewhere else (such as
  the carry flag). 
  <p>

  Also, some systems allow both logical and arithmetic shifts. In a 
  logical shift the whole byte, register or word is involved. In 
  arithmetic, the high bit recognized as a sign flag is not shifted. 
  <p>

  Arithmetic - add, subtract, multiply, divide. On the single register
  integer level, these features can be accomplished with the boolean, 
  single operation, and shift functions. However, many systems handle 
  multi-register values, floats, and BCD numbers. Additionally some 
  systems provide additonal features such as trig. functions for 
  graphic calculations.
  <p>
</ul>

<b>Stack manipulation</b> - push, pop, swap, rotate. On some systems that 
provide a stack, the stack is used for holding data other than the return 
address of a call. These may include using the stack as a scratch area for 
complex mathematical evaluatons or free a register for temporary use without
loosing the data current stored in it.
<p>

For these to work, the stack has to be hardware (ISA) supported usually with
a dedicated regiser. 

<ul>
  Push places a value on the stack. 
  <p>

  Pop removes a value. 
  <p>

  Swap switches the last to values added. 
  <p>

  Rotate (with a possible value) takes each value and moves it down one 
  and moves the last value to the top.
</ul>