Revision as of 23:22, 29 December 2010

PAL Tutorial - An arbitrary-precision calculator

In release 0.7.x, only the PROSE Assembly Language (PAL) is available, and then only a subset of those instructions. So be aware, it's very low-level programming at this time. To learn more about the PROSE Programming Language, visit http://prose.sourceforge.net.

It is suggested you begin with the following articles before attempting this tutorial.

• Introduction to PROSE
• Quick start guide
• Running your first program
• Short tour of PAL

A full list of available tutorials for the PROSE Assembly Language can be found on the tutorials index page.

This tutorial works through an example PAL program that implements a simple arbitrary-precision calculator.

Objectives

The objective is to write a PAL program that will allow for simple two-operand calculations on integers, floating-point numbers and rational numbers. This will demonstrate the use of variables and functions as well as basic integration with the GNU MP (Multi-Precision) library. Note that this is based on GMP 4.3.2.

The program will understand the following commands supplied on the standard input:

    M+N             adds two numbers M and N and displays the result
    M*N             multiplies two numbers M and N and displays the result
    M-N             subtracts two numbers M and N and displays the result
    M/N             divides two numbers M and N and displays the result
    M~N             divides two numbers M and N and displays the result
                        (when the number type is rational)
    int             set number type to integer
    flt             set number type to floating-point
    rat             set number type to rational
    help            displays this help page
    debug           toggles debug mode ON and OFF
    quit            exits this program

Integers, floating-point numbers and rational numbers will all take the form accepted by the GNU MP library. Floating-point numbers at this time have a default precision set to at least 64 bits. Rational numbers are represented by X/Y where / separates the numerator from the denominator. Because of this, the ~ symbol will be used by the program for requesting a division operation with rational numbers.

Step 1 - Displaying a title

The first step is to display a title when the program is launched. This should be straightforward if you have followed previous tutorials:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% Simple arbitrary-precision calculator
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
._init
func/def	[main],     &[.main]
func/def	[title],    &[.func_title]
local/rtn

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% main()
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
.main
func/call	NULL, [title]
func/rtn

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% title()
%
% Display title of program to stderr
%
% Arguments:
%	None
%
% Returns:
%	None
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
.func_title
attr/mod	![.prose.sys.io], [psStreamError], [
pscalc: a simple arbitrary-precision calculator
Type 'help' for a list of commands or 'quit' to exit
]
func/rtn

Step 2 - Accepting basic commands

The help and quit commands should be easy to implement. We need a function which we'll call getcmd() that reads the next command from standard input and strips the trailing newline character. This demonstrates returning a value from a function, and introduces us to the concept of the encoded attribute value (or XVALUE). Here is the function code:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% getcmd()
%
% Gets next command from stdin (stripping newline character)
%
% Arguments:
%	None
%
% Returns:
%	String, or empty string if input is exhausted
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
.func_getcmd
reg/load	P15, ![.prose.sys.io]

% Display command prompt to stderr
attr/mod	P15, [psStreamError], [> ]

% Collect input
attr/copy	P0, P15, [psStreamIn]
reg/jmpeq	&[.getcmd_null], P0, NULL

% Strip trailing newline character
reg/load	A, (P0)
op/decr
error/jmp	&[.no_strip], ![.prose.error.sys.OutOfRange]
reg/load	P2, (P0, A)
op/shr		P2, P2, #24
reg/jmpneq	&[.no_strip], P2, #10
reg/save	P0, (A)
func/rtn	P0

.no_strip
error/clr
func/rtn	P0

.getcmd_null
func/rtn	[]

The only difference in the above code from previous tutorials is that a string is being passed to the func/rtn instruction. This necessitates an additional argument to the corresponding func/def instruction which we insert into the ._init section. We'll re-write that section now to pre-load some attribute definitions into registers P0 and P1:

._init
attr/load	P0, [psString], P1, [psIndex]

%
% Function definitions
%
func/def	[main],     &[.main]
func/def	[title],    &[.func_title]
func/def	[getcmd],   &[.func_getcmd],	P0
local/rtn

Here the psString attribute is being passed to the 3rd argument of func/def. This identifies the type of data that the function will return. If omitted, or NULL, then the function cannot return a value. Function return values can only be a variable type, i.e. a type that can be assigned to a program variable. We discuss variable types later on in this tutorial.

When the getcmd() function is called, it will return the next line from standard input. All functions that return values will return them as encoded attribute values. This is data that is encoded for a specific attribute type, in this case psString. It will be saved into a register as the type PSUNIT_TYPE_XVALUE. When the func/rtn instruction is used with a value, that value can be provided in the correct encoding (in which case it is simply returned as-is), or if not it will be re-encoded automatically by the PROSE engine before the return completes. This re-encoding will usually take place via a byte string representation, because all variable types can be represented in byte string format.

This concept is worth demonstrating. Before continuing, try calling the getcmd() function and dumping the result, e.g. after a call to title() but before the main() function returns:

func/call	P0, [getcmd]
reg/dump	P0
obj/dump	P0

Notice the first argument to func/call. In previous tutorials this has been NULL. By providing a register you are specifying where the return value should be stored. Executing our program at this stage and typing the quit command should produce the following output:

pscalc: a simple arbitrary-precision calculator
Type 'help' for a list of commands or 'quit' to exit
> quit
register: P0
type: PSUNIT_TYPE_XVALUE (0x15)
psString

quit

The reg/dump instruction informs us that register P0 contains an encoded attribute value. The value is encoded according to the psString syntax. Essentially this is a value detached from an object attribute, but which could be attached at a later time. The value reported by obj/dump is 'quit' as expected, because this is the command you typed.

Remove the three lines added above and let's add proper support for the help and quit commands.

We'll use a data segment to map commands to an appropriate code label that can handle the command. This data segment can appear anywhere in the code, but we can put it ahead of the .main section:

%
% Mapping of code labels to commands
%
~cmdlist
EQUP { &[.quit] }; EQUS { [] }
EQUP { &[.quit] }; EQUS { [quit] }
EQUP { &[.help] }; EQUS { [help] }

We want empty input or the quit command to be routed to the .quit label, while the help command should be routed to the .help label. This requires a loop in the main() function after the title has been displayed:

.main
func/call	NULL, [title]

.loop
%
% Fetch command from stdin
%
func/call	P0, [getcmd]

%
% Test for one of the known commands
%
reg/load	P1, ( &[~cmdlist] )

.cmdloop
reg/load	P2, (P1)
reg/jmpeq	&[.cmdloop_end], P2, NULL
reg/load	P3, (P1)
reg/jmpneq	&[.cmdloop], P0, P3
reg/clr		P1
local/jmp	P2

.cmdloop_end
% No known command, just ignore and loop for now ...
reg/clr		P0, P1
local/jmp	&[.loop]

This uses the reg/load instruction in one of its indirect addressing modes, in order to iterate the values within a data segment. This is described in the reg_load_segment(5) man page.

The .quit and .help sections can follow immediately on:

.quit
% Exit program
reg/clr		P0
func/rtn

.help
% Display help message
reg/clr		P0
attr/mod	![.prose.sys.io], [psStreamError], [
M+N		adds two numbers M and N and displays the result
M*N		multiplies two numbers M and N and displays the result
M-N		subtracts two numbers M and N and displays the result
M/N		divides two numbers M and N and displays the result
M~N		divides two numbers M and N and displays the result
			(when the number type is rational)
int		set number type to integer
flt		set number type to floating-point
rat		set number type to rational
help		displays this help page
debug		toggles debug mode ON and OFF
quit		exits this program
]
local/jmp	&[.loop]

Notice we're choosing to display informational messages to psStreamError (the standard error stream). This is to keep the standard output clear for the mathematical results so that commands can be piped into the program and results extracted easily by an external command.

Step 3 - Switching number types

This calculator will support integers, floating-points and rationals. These are specified by the attribute types psInteger, psFloat and psRational. The calculator will accept the commands int, flt and rat to switch between these types.

We'll use a global variable to store the current number type. Program variables in PAL are objects in the nexus, and as any other object in the nexus they are defined using classes and attributes. Typically a variable has the psVariable class as well as one other variable-type class, and a variable-type attribute containing the value. The variable-type classes and variable-type attributes are named identically and are listed in the ps_attributes(5) and ps_classes(5) man pages. Those which we will be using are:

• psFloat - GMP floating-point number
• psIndex - raw index (32-bit unsigned integer)
• psInteger - GMP integer
• psRational - GMP rational number
• psString - string data

Ordinary object create commands could be used for creating program variables, e.g. obj/def, class/add, attr/add and obj/commit. However, for convenience, the PAL instruction set comes with some short-cut instructions for creating variables:

• var/local - create a local variable
• var/static - create a static variable
• var/global - create a global variable
• var/def - create a variable in an arbitrary location

These will assign the correct classes and attributes to the object and commit it to the nexus. The only difference between the above instructions is the location in which the variable object is created.

Resources from this tutorial

• Source code: pscalc.pal

Further reading

See the other tutorials available for the PROSE Assembly Language on the tutorials index page.

PROSE is released with detailed manual pages that describe how PAL operates, and how each instruction is used. These manual pages can be read using the man command, for example man pal_intro or man pal_commands, or from the project links on the main page of this wiki.