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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Lesson 9 Screen04</title>
<link href="stylesheet.css" rel="Stylesheet" type="text/css" />
<script language="javascript" type="text/javascript" src="script.js"></script>
</head>
<body>
<div id="contentAll">
<div id="courseHead">
<h1>
Lesson 9 Screen04</h1>
</div>
<div id="pageAll">
<div id="pageBody">
<p>
The Screen04 lesson builds on Screen03, by teaching how to manipulate text. It is
assumed you have the code for the <a href="screen03.html">Lesson 8: Screen03</a>
operating system as a basis.</p>
<div class="ucampas-toc">
</div>
<h2 id="stringmanipulation">
1 String Manipulation</h2>
<div class="informationBox"><p>Variadic functions look much less intuitive in assembly code. Nevertheless,
they are useful and powerful concepts.</p></div>
<p>
Being able to draw text is lovely, but unfortunately at the moment you can only
draw strings which are already prepared. This is fine for displaying something like
the command line, but ideally we would like to be able to display and text we so
desire. As per usual, if we put the effort in and make an excellent function that
does all the string manipulation we could ever want, we get much easier code later
on in return. Once such complicated function in C programming is sprintf. This function
generates a string based on a description given as another string and additional
arguments. What is interesting about this function is that it is variadic. This
means that it takes a variable number of parameters. The number of parameters depends
on the exact format string, and so cannot be determined in advance.</p>
<p>
The full function has many options, and I list a few here. I've highlighted the
ones which we will implement in this tutorial, though you can try to implement more.</p>
<p>
The function works by reading the format string, and then interpreting it using
the table below. Once an argument is used, it is not considered again. The return
value of the function is the number of characters written. If the method fails,
a negative number is returned.</p>
<table>
<caption>
Table 1.1 sprintf formatting rules</caption>
<thead>
<tr>
<th>
Sequence
</th>
<th>
Meaning
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellBlue">
<em>Any character except %</em>
</td>
<td>
Copies the character to the output.
</td>
</tr>
<tr>
<td class="cellBlue">
%%
</td>
<td>
Writes a % character to the output.
</td>
</tr>
<tr>
<td class="cellBlue">
%c
</td>
<td>
Writes the next argument as a character.
</td>
</tr>
<tr>
<td class="cellBlue">
%d <em>or</em> %i
</td>
<td>
Writes the next argument as a base 10 signed integer.
</td>
</tr>
<tr>
<td>
%e
</td>
<td>
Writes the next argument in scientific notation using e<strong>N</strong> to mean
×10<sup><strong>N</strong></sup>.
</td>
</tr>
<tr>
<td>
%E
</td>
<td>
Writes the next argument in scientific notation using E<strong>N</strong> to mean
×10<sup><strong>N</strong></sup>.
</td>
</tr>
<tr>
<td>
%f
</td>
<td>
Writes the next argument as a decimal IEEE 754 floating point number.
</td>
</tr>
<tr>
<td>
%g
</td>
<td>
Same as the shorter of %e and %f.
</td>
</tr>
<tr>
<td>
%G
</td>
<td>
Same as the shorter of %E and %f.
</td>
</tr>
<tr>
<td class="cellBlue">
%o
</td>
<td>
Writes the next argument as a base 8 unsigned integer.
</td>
</tr>
<tr>
<td class="cellBlue">
%s
</td>
<td>
Writes the next argument as if it were a pointer to a null terminated string.
</td>
</tr>
<tr>
<td class="cellBlue">
%u
</td>
<td>
Writes the next argument as a base 10 unsigned integer.
</td>
</tr>
<tr>
<td class="cellBlue">
%x
</td>
<td>
Writes the next argument as a base 16 unsigned integer, with lowercase a,b,c,d,e
and f.
</td>
</tr>
<tr>
<td>
%X
</td>
<td>
Writes the next argument as a base 16 unsigned integer, with uppercase A,B,C,D,E
and F.
</td>
</tr>
<tr>
<td>
%p
</td>
<td>
Writes the next argument as a pointer address.
</td>
</tr>
<tr>
<td class="cellBlue">
%n
</td>
<td>
Writes nothing. Copies instead the number of characters written so far to the location
addressed by the next argument.
</td>
</tr>
</tbody>
</table>
<p>
Further to the above, many additional tweaks exist to the sequences, such as specifying
minimum length, signs, etc. More information can be found at <a href="http://www.cplusplus.com/reference/clibrary/cstdio/sprintf/">
sprintf - C++ Reference</a>.</p>
<p>
Here are a few examples of calls to the method and their results to illustrate its
use.</p>
<table>
<caption>
Table 1.2 sprintf example calls</caption>
<thead>
<tr>
<th>
Format String
</th>
<th>
Arguments
</th>
<th>
Result
</th>
</tr>
</thead>
<tbody>
<tr>
<td>
"%d"
</td>
<td>
13
</td>
<td>
"13"
</td>
</tr>
<tr>
<td>
"+%d degrees"
</td>
<td>
12
</td>
<td>
"+12 degrees"
</td>
</tr>
<tr>
<td>
"+%x degrees"
</td>
<td>
24
</td>
<td>
"+1c degrees"
</td>
</tr>
<tr>
<td>
"'%c' = 0%o"
</td>
<td>
65, 65
</td>
<td>
"'A' = 0101"
</td>
</tr>
<tr>
<td>
"%d * %d%% = %d"
</td>
<td>
200, 40, 80
</td>
<td>
"200 * 40% = 80"
</td>
</tr>
<tr>
<td>
"+%d degrees"
</td>
<td>
-5
</td>
<td>
"+-5 degrees"
</td>
</tr>
<tr>
<td>
"+%u degrees"
</td>
<td>
-5
</td>
<td>
"+4294967291 degrees"
</td>
</tr>
</tbody>
</table>
<p>
Hopefully you can already begin to see the usefulness of the function. It does take
a fair amount of work to program, but our reward is a very general function we can
use for all sorts of purposes.</p>
<h2 id="division">
2 Division</h2>
<div class="informationBox"><p>Division is the slowest and most complicated of the basic
mathematical operators. It is not implemented directly in ARM assembly code because it takes so
long to deduce the answer, and so isn't a 'simple' operation.</p></div>
<p>
While this function does look very powerful, it also looks very complicated. The
easiest way to deal with its many cases is probably to write functions to deal with
some common tasks it has. What would be useful would be a function to generate the
string for a signed and an unsigned number in any base. So, how can we go about
doing that? Try to devise an algorithm quickly before reading on.</p>
<p>
The easiest way is probably the exact way I mentioned in <a href="ok01.html">Lesson 1:
OK01</a>, which is the division remainder method. The idea is the following:</p>
<ol>
<li>Divide the current value by the base you're working in.</li>
<li>Store the remainder.</li>
<li>If the new value is not 0, go to 1.</li>
<li>Reverse the order of the remainders. This is the answer.</li></ol>
<p>
For example:</p>
<table>
<caption>
Table 2.1 Example base 2 conversion</caption>
<thead>
<tr>
<th>
Value
</th>
<th>
New Value
</th>
<th>
Remainder
</th>
</tr>
</thead>
<tbody>
<tr>
<td>
137
</td>
<td>
68
</td>
<td>
1
</td>
</tr>
<tr>
<td>
68
</td>
<td>
34
</td>
<td>
0
</td>
</tr>
<tr>
<td>
34
</td>
<td>
17
</td>
<td>
0
</td>
</tr>
<tr>
<td>
17
</td>
<td>
8
</td>
<td>
1
</td>
</tr>
<tr>
<td>
8
</td>
<td>
4
</td>
<td>
0
</td>
</tr>
<tr>
<td>
4
</td>
<td>
2
</td>
<td>
0
</td>
</tr>
<tr>
<td>
2
</td>
<td>
1
</td>
<td>
0
</td>
</tr>
<tr>
<td>
1
</td>
<td>
0
</td>
<td>
1
</td>
</tr>
</tbody>
</table>
<p>
So the answer is 10001001<sub>2</sub></p>
<p>
The unfortunate part about this procedure is that it unavoidably uses division.
Therefore, we must first contemplate division in binary.</p>
<p>
For a refresher on long division expand the box below.</p>
<div class="expandableHeader" onclick="return ExpandToggle('longdiv')"><p><a class="icon-more">Long division explained</a></p></div>
<div id="longdiv" class="expandable">
<p>
Let's suppose we wish to divide 4135 by 17.</p>
<pre> <span style="text-decoration: underline;"> <span style="color: Olive;">0</span><span
style="color: red;">2</span><span style="color: green;">4</span><span style="color: blue;">3</span></span> r 4
17)<span style="color: Olive;">4</span>135
<span style="color: Olive;">0 </span> <span style="color: Olive;">0</span> × 17 = <span
style="color: Olive;">0</span>000
<span style="color: red;">41</span>35 4135 - <span style="color: Olive;">0</span> = 4135
<span style="color: red;">34 </span> <span style="color: red;">2</span>00 × 17 = <span
style="color: red;">34</span>00
<span style="color: green;">73</span>5 4135 - <span style="color: red;">3400</span> = 735
<span style="color: green;">68 </span> <span style="color: green;">4</span>0 × 17 = <span
style="color: green;">68</span>0
<span style="color: blue;">55</span> 735 - <span style="color: green;">680</span> = 55
<span style="color: blue;">51</span> <span style="color: blue;">3</span> × 17 = <span
style="color: blue;">51</span>
4 55 - <span style="color: blue;">51</span> = 4
Answer: 243 remainder 4</pre>
<p>
First of all we would look at the top digit of the dividend. We see that the smallest
multiple of the divisor which is less or equal to it is 0. We output a 0 to the
result.</p>
<p>
Next we look at the second to top digit of the dividend and all higher digits. We
see the smallest multiple of the divisor which is less than or equal is 34. We output
a 2 and subtract 3400.</p>
<p>
Next we look at the third digit of the dividend and all higher digits. The smallest
multiple of the divisor that is less than or equal to this is 68. We output 4 and
subtract 680.</p>
<p>
Finally we look at all remaining digits. We see that the lowest multiple of the
divisor that is less than the remaining digits is 51. We output a 3, subtract 51.
The result of the subtraction is our remainder.</p>
</div>
<p>
To implement division in assembly code, we will implement binary long division.
We do this because the numbers are stored in binary, which gives us easy access
to the all important bit shift operations, and because division in binary is simpler
than in any higher base due to the much lower number of cases.</p>
<pre style="float: left;"> <span style="text-decoration: underline;"> 1011</span> r 1
1010)1101111
1010
11111
1010
1011
1010
1</pre>
<p>
This example shows how binary long division works. You simply shift the divisor
as far right as possible without exceeding the dividend, output a 1 according to
the poisition and subtract the number. Whatever remains is the remainder. In this
case we show 1101111<sub>2</sub> ÷ 1010<sub>2</sub> = 1011<sub>2</sub> remainder
1<sub>2</sub>. In decimal, 111 ÷ 10 = 11 remainder 1.</p>
<p style="clear: left;">
Try to implement long division yourself now. You should write a function, DivideU32
which divides r0 by r1, returning the result in r0, and the remainder in r1. Below,
we will go through a very efficient implementation.
</p>
<div class="pseudoCodeBlock"><p>
<span class="keyword">function </span>DivideU32(r0 <span class="keyword">is</span>
dividend, r1 <span class="keyword">is</span> divisor)<br />
<div class="indent"><p>
<span class="keyword">set</span> shift <span class="keyword">to</span> 31<br />
<span class="keyword">set</span> result <span class="keyword">to</span> 0<br />
<span class="keyword">while</span> shift ≥ 0<br />
<div class="indent"><p>
<span class="keyword">if</span> dividend ≥ (divisor << shift) <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> dividend <span class="keyword">to</span> dividend
- (divisor << shift)<br />
<span class="keyword">set</span> result <span class="keyword">to</span> result +
1<br />
</div><p>
<span class="keyword">end if</span><br />
<span class="keyword">set</span> result <span class="keyword">to</span> result <<
1<br />
<span class="keyword">set</span> shift <span class="keyword">to</span> shift - 1<br />
</div><p>
<span class="keyword">loop</span><br />
<span class="keyword">return</span> (result, dividend)
</div><p>
<span class="keyword">end function</span><br />
</div>
<p>
This code does achieve what we need, but would not work as assembly code. Our problem
comes from the fact that our registers only hold 32 bits, and so the result of <span
class="pseudoCodeInline">divisor << shift</span> may not fit in a register
(we call this overflow). This is a real problem. Did your solution have overflow?</p>
<p>
Fortunately, an instruction exists called <span class="armCodeInline">clz</span>
or count leading zeros, which counts the number of zeros in the binary representation
of a number starting at the top bit. Conveniently, this is exactly the number of
times we can shift the register left before overflow occurs. Another optimisation
you may spot is that we compute <span class="pseudoCodeInline">divisor << shift</span>
twice each loop. We could improve upon this by shifting the divisor at the beginning,
then shifting it down at the end of each loop to avoid any need to shift it elsewhere.</p>
<p>
Let's have a look at the assembly code to make further improvements.</p>
<div class="armCodeBlock"><p>
.globl DivideU32<br />
DivideU32:<br />
result .req r0<br />
remainder .req r1<br />
shift .req r2<br />
current .req r3<br />
<br />
clz shift,r1<br />
lsl current,r1,shift<br />
mov remainder,r0<br />
mov result,#0<br />
<br />
divideU32Loop$:<br />
<div class="indent"><p>
cmp shift,#0<br />
blt divideU32Return$<br />
cmp remainder,current<br />
<br />
addge result,result,#1<br />
subge remainder,current<br />
sub shift,#1<br />
lsr current,#1<br />
lsl result,#1<br />
b divideU32Loop$<br />
</div><p>
<br />
divideU32Return$:<br />
.unreq current<br />
mov pc,lr<br />
<br />
.unreq result<br />
.unreq remainder<br />
.unreq shift<br />
</div>
<div class="commandBox"><p>
<span class="armCodeInline">clz dest,src</span> stores the number of zeros from
the top to the first one of register <span class="armCodeInline">dest</span> to
register <span class="armCodeInline">src</span>
</div>
<p>
You may, quite rightly, think that this looks quite efficient. It is pretty good,
but division is a very expensive operation, and one we may wish to do quite often,
so it would be good if we could improve the speed in any way. When looking to optimise
code with a loop in it, it is always important to consider how many times the loop
must run. In this case, the loop will run a maximum of 31 times for an input of
1. Without making special cases, this could often be improved easily. For example
when dividing 1 by 1, no shift is required, yet we shift the divisor to each of
the positions above it. This could be improved by simply using the new <span class="armCodeInline">
clz</span> command on the dividend and subtracting this from the shift. In the
case of 1 ÷ 1, this means shift would be set to 0, rightly indicating no
shift is required. If this causes the shift to be negative, the divisor is bigger
than the dividend and so we know the result is 0 remainder the dividend. Another
quick check we could make is if the current value is ever 0, then we have a perfect
division and can stop looping.</p>
<div class="armCodeBlock"><p>
.globl DivideU32<br />
DivideU32:<br />
result .req r0<br />
remainder .req r1<br />
shift .req r2<br />
current .req r3<br />
<br />
clz shift,r1<br />
clz r3,r0<br />
subs shift,r3<br />
lsl current,r1,shift<br />
mov remainder,r0<br />
mov result,#0<br />
blt divideU32Return$<br />
<br />
divideU32Loop$:<br />
<div class="indent"><p>
cmp remainder,current<br />
blt divideU32LoopContinue$<br />
<br />
add result,result,#1<br />
subs remainder,current<br />
lsleq result,shift
<br />
beq divideU32Return$<br />
</div><p>
divideU32LoopContinue$:<br />
<div class="indent"><p>
subs shift,#1<br />
lsrge current,#1<br />
lslge result,#1<br />
bge divideU32Loop$<br />
</div><p>
<br />
divideU32Return$:<br />
.unreq current<br />
mov pc,lr<br />
<br />
.unreq result<br />
.unreq remainder<br />
.unreq shift<br />
</div>
<p>
Copy the code above to a file called 'maths.s'.</p>
<h2 id="numberstrings">
3 Number Strings</h2>
<p>
Now that we can do division, let's have another look at implementing number to string
conversion. The following is pseudo code to convert numbers from registers into strings
in up to base 36. By convention, a % b means the remainder of dividing a by b.</p>
<div class="pseudoCodeBlock"><p>
<span class="keyword">function</span> SignedString(r0 <span class="keyword">is</span>
value, r1 <span class="keyword">is</span> dest, r2 <span class="keyword">is</span>
base)<br />
<div class="indent"><p>
<span class="keyword">if</span> value ≥ 0<br />
<span class="keyword">then return</span> UnsignedString(value, dest, base)<br />
<span class="keyword">otherwise</span><br />
<div class="indent"><p>
<span class="keyword">if</span> dest > 0 <span class="keyword">then</span><br />
<div class="indent"><p>
setByte(dest, '-')<br />
<span class="keyword">set</span> dest <span class="keyword">to</span> dest + 1<br />
</div><p>
<span class="keyword">end if</span><br />
<span class="keyword">return</span> UnsignedString(-value, dest, base) + 1<br />
</div><p>
<span class="keyword">end if</span><br />
</div><p>
<span class="keyword">end function</span><br />
<br />
<span class="keyword">function</span> UnsignedString(r0 <span class="keyword">is</span>
value, r1 <span class="keyword">is</span> dest, r2 <span class="keyword">is</span>
base)<br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> 0<br />
<span class="keyword">do</span><br />
<div class="indent"><p>
<span class="keyword">set</span> (value, rem) <span class="keyword">to</span> DivideU32(value,
base)<br />
<span class="keyword">if</span> rem > 10<br />
<span class="keyword">then set</span> rem <span class="keyword">to</span> rem +
'0'<br />
<span class="keyword">otherwise set</span> rem <span class="keyword">to</span> rem
- 10 + 'a'<br />
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> setByte(dest + length, rem)<br />
<span class="keyword">set</span> length <span class="keyword">to</span> length +
1<br />
</div><p>
<span class="keyword">while</span> value > 0<br />
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> ReverseString(dest, length)<br />
<span class="keyword">return</span> length<br />
</div><p>
<span class="keyword">end function</span><br />
<br />
<span class="keyword">function</span> ReverseString(r0 <span class="keyword">is</span>
string, r1 <span class="keyword">is</span> length)<br />
<div class="indent"><p>
<span class="keyword">set</span> end <span class="keyword">to</span> string + length
- 1<br />
<span class="keyword">while</span> end > start<br />
<div class="indent"><p>
<span class="keyword">set</span> temp1 <span class="keyword">to</span> readByte(start)<br />
<span class="keyword">set</span> temp2 <span class="keyword">to</span> readByte(end)<br />
setByte(start, temp2)<br />
setByte(end, temp1)<br />
<span class="keyword">set</span> start <span class="keyword">to</span> start + 1<br />
<span class="keyword">set</span> end <span class="keyword">to</span> end - 1<br />
</div><p>
<span class="keyword">end while</span><br />
</div><p>
<span class="keyword">end function</span><br />
</div>
<p>
In a file called 'text.s' implement the above. Remember that if you get stuck, a
full solution can be found on the downloads page.</p>
<h2 id="formatstrings">
4 Format Strings</h2>
<p>
Let's get back to our string formatting method. Since we're programming our own
operating system, we can add or change formatting rules as we please. We may find
it useful to add a %b operation that outputs a number in binary, and if you're not
using null terminated strings, you may wish to alter the behaviour of %s to take
the length of the string from another argument, or from a length prefix if you wish.
I will use a null terminator in the example below.</p>
<p>
One of the main obstacles to implementing this function is that the number of arguments
varies. According to the ABI, additional arguments are pushed onto the stack before
calling the method in reverse order. So, for example, if we wish to call our method
with 8 parameters; 1,2,3,4,5,6,7 and 8, we would do the following:</p>
<ol>
<li>Set r0 = 5, r1 = 6, r2 = 7, r3 = 8</li>
<li>Push {r0,r1,r2,r3}</li>
<li>Set r0 = 1, r1 = 2, r2 = 3, r3 = 4</li>
<li>Call the function</li>
<li>Add sp,#4*4</li>
</ol>
<p>
Now we must decide what arguments our function actually needs. In my case, I used
the format string address in r0, the length of the format string in r1, the destination
string address in r2, followed by the list of arguments required, starting in r3
and continuing on the stack as above. If you wish to use a null terminated format
string, the parameter in r1 can be removed. If you wish to have a maximum buffer
length, you could store this in r3. As an additional modification, I think it is
useful to alter the function so that if the destination string address is 0, no
string is outputted, but an accurate length is still returned, so that the length
of a formatted string can be accurately determined.</p>
<p>
If you wish to attempt the implementation on your own, try it now. If not, I will
first construct the pseudo code for the method, then give the assembly code implementation.</p>
<div class="pseudoCodeBlock"><p>
<span class="keyword">function</span> StringFormat(r0 <span class="keyword">is</span>
format, r1 <span class="keyword">is</span> formatLength, r2 <span class="keyword">is</span>
dest, ...)<br />
<div class="indent"><p>
<span class="keyword">set</span> index <span class="keyword">to</span> 0<br />
<span class="keyword">set</span> length <span class="keyword">to</span> 0<br />
<span class="keyword">while</span> index < formatLength<br />
<div class="indent"><p>
<span class="keyword">if</span> readByte(format + index) = '%' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> index <span class="keyword">to</span> index + 1<br />
<span class="keyword">if</span> readByte(format + index) = '%' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> setByte(dest + length, '%')<br />
<span class="keyword">set</span> length <span class="keyword">to</span> length +
1<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'c' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> setByte(dest + length, nextArg)<br />
<span class="keyword">set</span> length <span class="keyword">to</span> length +
1<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'd' <span class="keyword">
or</span> 'i' <span class="keyword">then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> length +
SignedString(nextArg, dest, 10)<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'o' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> length +
UnsignedString(nextArg, dest, 8)<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'u' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> length +
UnsignedString(nextArg, dest, 10)<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'b' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> length +
UnsignedString(nextArg, dest, 2)<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'x' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> length <span class="keyword">to</span> length +
UnsignedString(nextArg, dest, 16)<br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 's' <span class="keyword">
then</span><br />
<div class="indent"><p>
<span class="keyword">set</span> str <span class="keyword">to</span> nextArg<br />
<span class="keyword">while</span> getByte(str) != '\0'<br />
<div class="indent"><p>
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> setByte(dest + length, getByte(str))<br />
<span class="keyword">set</span> length <span class="keyword">to</span> length +
1<br />
<span class="keyword">set</span> str <span class="keyword">to</span> str + 1<br />
</div><p>
<span class="keyword">loop</span><br />
</div><p>
<span class="keyword">otherwise if</span> readByte(format + index) = 'n' <span class="keyword">
then</span><br />
<div class="indent"><p>
setWord(nextArg, length)<br />
</div><p>
<span class="keyword">end if</span><br />
</div><p>
<span class="keyword">otherwise</span><br />
<div class="indent"><p>
<span class="keyword">if</span> dest > 0<br />
<span class="keyword">then</span> setByte(dest + length, readByte(format + index))<br />
<span class="keyword">set</span> length <span class="keyword">to</span> length +
1<br />
</div><p>
<span class="keyword">end if</span><br />
<span class="keyword">set</span> index <span class="keyword">to</span> index + 1<br />
</div><p>
<span class="keyword">loop</span><br />
<span class="keyword">return </span>length<br />
</div><p>
<span class="keyword">end function</span><br />
</div>
<p>
Although this function is massive, it is quite straightforward. Most of the code
goes into checking all the various conditions, the code for each one is simple.
Further, all the various unsigned integer cases are the same but for the base, and
so can be summarised in assembly. This is given below.</p>
<div class="armCodeBlock"><p>
.globl FormatString<br />
FormatString:<br />
format .req r4<br />
formatLength .req r5<br />
dest .req r6<br />
nextArg .req r7<br />
argList .req r8<br />
length .req r9<br />
<br />
push {r4,r5,r6,r7,r8,r9,lr}<br />
mov format,r0<br />
mov formatLength,r1<br />
mov dest,r2<br />
mov nextArg,r3<br />
add argList,sp,#7*4<br />
mov length,#0<br />
<br />
formatLoop$:<br />
<div class="indent"><p>
subs formatLength,#1<br />
movlt r0,length<br />
poplt {r4,r5,r6,r7,r8,r9,pc}<br />
<br />
ldrb r0,[format]<br />
add format,#1<br />
teq r0,#'%'<br />
beq formatArg$<br />
<br />
</div><p>
formatChar$:<br />
<div class="indent"><p>
teq dest,#0<br />
strneb r0,[dest]<br />
addne dest,#1<br />
add length,#1<br />
b formatLoop$<br />
<br />
</div><p>
formatArg$:<br />
<div class="indent"><p>
subs formatLength,#1<br />
movlt r0,length<br />
poplt {r4,r5,r6,r7,r8,r9,pc}<br />
<br />
ldrb r0,[format]<br />
add format,#1<br />
teq r0,#'%'<br />
beq formatChar$<br />
<br />
teq r0,#'c'<br />
moveq r0,nextArg<br />
ldreq nextArg,[argList]<br />
addeq argList,#4<br />
beq formatChar$<br />
<br />
teq r0,#'s'<br />
beq formatString$<br />
<br />
teq r0,#'d'<br />
beq formatSigned$<br />
<br />
teq r0,#'u'<br />
teqne r0,#'x'<br />
teqne r0,#'b'<br />
teqne r0,#'o'<br />
beq formatUnsigned$<br />
<br />
b formatLoop$<br />
<br />
</div><p>
formatString$:<br />
<div class="indent"><p>
ldrb r0,[nextArg]<br />
teq r0,#0x0<br />
ldreq nextArg,[argList]<br />
addeq argList,#4<br />
beq formatLoop$<br />
add length,#1<br />
teq dest,#0<br />
strneb r0,[dest]<br />
addne dest,#1<br />
add nextArg,#1<br />
b formatString$<br />
<br />
</div><p>
formatSigned$:<br />
<div class="indent"><p>
mov r0,nextArg<br />
ldr nextArg,[argList]<br />
add argList,#4<br />
mov r1,dest<br />
mov r2,#10<br />
bl SignedString<br />
teq dest,#0<br />
addne dest,r0<br />
add length,r0<br />
b formatLoop$<br />
<br />
</div><p>
formatUnsigned$:<br />
<div class="indent"><p>
teq r0,#'u'<br />
moveq r2,#10<br />
teq r0,#'x'<br />
moveq r2,#16<br />
teq r0,#'b'<br />
moveq r2,#2<br />
teq r0,#'o'<br />
moveq r2,#8<br />
<br />
mov r0,nextArg<br />
ldr nextArg,[argList]<br />
add argList,#4<br />
mov r1,dest<br />
bl UnsignedString<br />
teq dest,#0<br />
addne dest,r0<br />
add length,r0<br />
b formatLoop$<br />
</div><p>
</div>
<h2 id="convertos">
5 Convert OS</h2>
<p>
Feel free to try using this method however you wish. As an example, here is the
code to generate a conversion chart from base 10 to binary to hexadecimal to octal
and to ASCII.</p>
<p>
Delete all code after <span class="armCodeInline">bl SetGraphicsAddress</span> in
'main.s' and replace it with the following:</p>
<div class="armCodeBlock"><p>
mov r4,#0<br />
loop$:<br />
ldr r0,=format<br />
mov r1,#formatEnd-format<br />
ldr r2,=formatEnd<br />
lsr r3,r4,#4<br />
push {r3}<br />
push {r3}<br />
push {r3}<br />
push {r3}<br />
bl FormatString<br />
add sp,#16<br />
<br />
mov r1,r0<br />
ldr r0,=formatEnd<br />
mov r2,#0<br />
mov r3,r4<br />
<br />
cmp r3,#768-16<br />
subhi r3,#768<br />
addhi r2,#256<br />
cmp r3,#768-16<br />
subhi r3,#768<br />