The S/370 machine instructions are divided into two broad
categories: those which require the CPU be in supervisor state,
and those that don't. The instructions which do not require
supervisor state are normally referred to as "problem state"
instructions. As you might recall, we said that bit 15 of
the S/370 PSW controls which state (supervisor or problem)
the CPU is in.
Generally speaking, most operating systems execute the user's
program in problem state, with VM's CMS operating system being
a notable exception. Running the user's program in problem
state prevents one user's program from adversely affecting
another user's program and data, or the operating system's
programs and data.
The MVS operating system, which normally executes the user's
program in problem state, goes to great lengths to prevent
unauthorized entry into supervisor state. The only defined
interface MVS provides for a user program to enter supervisor
state is called the Authorized Program Facility (APF). Normally,
only systems programmers write programs that use this APF facility.
Programs that are running in problem state are restricted to
executing problem state instructions. Programs that are running
in supervisor state may execute either problem program instructions
or supervisor state instructions. That is to say, supervisor state
programs may execute all the valid instructions.
Before you're ready to start writing programs that execute in
supervisor state, you'll need to master most of the instructions
that execute in problem state.
Since we're going to lump several chapters into this section,
I've laid out how the S/370 hardware views the various categories
Problem state instructions
Chapter 7 - General Instructions
Chapter 8 - Decimal Instructions
Chapter 9 - Floating Point Instructions
Supervisor state instructions
Chapter 10 - Control Instructions
The category "General Instructions" is a bit broader than I've
indicated above; there are some instructions related to decimal
instructions in this category, largely having to do with conversion
between decimal and binary, and vice versa.
Additionally, there are some miscellaneous instructions included in
Chapter Seven that don't really fit anywhere else.
But in general, most of the instructions in Chapter 7 relate to
As you begin your assembly language coding career, Chapter Seven is
where you'll spend most of your time.
Sometimes even experienced programmers never open Chapter Ten, and
relatively few coders ever bother to read Chapter Nine. When you've
got a lot of floating point math to perform, most people select
another language in which to code.
At this point, I recommend everyone read the following sections in
Signed and Logical Comparison
You might have a little trouble understanding the material, but you'll
get at least some notion of the different ways different instructions
can treat data. Glance down the list of General Instructions at the
beginning of Chapter Seven, just to get a feel for what's included.
This tutorial will mostly focus on binary numbers; by the time you've
gotten through handling binary numbers you should be familiar enough
with POPs to figure out the decimal and floating point numbers by yourself.
Next, read the Decimal-Number Formats section in Chapter Eight,
and glance at the list of Decimal Instructions.
After that, you might glance at the Floating-Point Number Representation
section in Chapter Nine, and the list of Floating Point Instructions.
In Chapter Ten, you can briefly examine the unlabeled section before
the first Control Instruction: CONNECT CHANNEL SET.
Read down the list of instructions in the table, just to get a feel
for the types of control operations the S/370 provides.
Dealing with Binary Numbers
If you're a little rusty on dealing with Binary, Decimal, and/or
Hexadecimal numbers, this might be a good time to take a detour
Numbers page, which also
gives some introductory material on converting between
binary, decimal, and hexadecimal.
Before we leave our discussion of instructions, keep in mind that
we're just surveying POPs to get a sense of what's contained in
the manual, and where it might be located.
We've still got our IEFBR14 program to discuss, and we'll starting
using POPs just as soon as we get back to it.