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<!doctype html>
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<title>CS 2150: 07-ibcm slide set</title>
<meta name="description" content="A set of slides for a course on Program and Data Representation">
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<section data-markdown id="cover"><script type="text/template">
# CS 2150
### Program and Data Representation
<center><small>[Mark Floryan](http://www.cs.virginia.edu/~mrf8t) / [[email protected]](mailto:[email protected])</small></center>
<center><small>Repository: [github.com/markfloryan/pdr](http://github.com/markfloryan/pdr) / [↑](index.html) / <a href="13-esoteric-pls.html?print-pdf"><img class="print" width="20" src="images/print-icon.png"></a></small></center>
## IBCM (machine language)
</script></section>
<section>
<h2>CS 2150 Roadmap</h2>
<table class="wide">
<tr><td colspan="3"><p class="center">Data Representation</p></td><td></td><td colspan="3"><p class="center">Program Representation</p></td></tr>
<tr>
<td class="top"><small> <br> <br>string<br> <br> <br> <br>int x[3]<br> <br> <br> <br>char x<br> <br> <br> <br>0x9cd0f0ad<br> <br> <br> <br>01101011</small></td>
<!-- image adapted from http://openclipart.org/detail/3677/arrow-left-right-by-torfnase -->
<td><img class="noborder" src="images/red-double-arrow.png" height="500" alt="vertical red double arrow"></td>
<td class="top"> <br>Objects<br> <br>Arrays<br> <br>Primitive types<br> <br>Addresses<br> <br>bits</td>
<td> </td>
<td class="top"><small> <br> <br>Java code<br> <br> <br>C++ code<br> <br> <br>C code<br> <br> <br>x86 code<br> <br> <br>IBCM<br> <br> <br>hexadecimal</small></td>
<!-- image adapted from http://openclipart.org/detail/3677/arrow-left-right-by-torfnase -->
<td><img class="noborder" src="images/green-double-arrow.png" height="500" alt="vertical green double arrow"></td>
<td class="top"> <br>High-level language<br> <br>Low-level language<br> <br>Assembly language<br> <br>Machine code</td>
</tr>
</table>
</section>
<section data-markdown><script type="text/template">
# Contents
[Introduction](#/introduction)
[IBCM Description](#/ibcmdesc)
[Writing IBCM code](#/writingibcm)
[Conclusions](#/conclusions)
</script></section>
<section>
<section id="introduction" data-markdown><script type="text/template">
# Introduction
</script></section>
<section data-markdown><script type="text/template">
## Assembly Language
- Machine language
- Instructions represented as patterns of bits (0s and 1s) that can be understood and processed by a central processing unit (CPU)
- Program stored in main memory
- Assembly language
- Human-readable notation for the machine language used to control a specific computer architecture
- Assembler translates to bits
</script></section>
<section data-markdown><script type="text/template">
## Why Learn Assembly Language?
- Machine designers
- Compiler writers
- Programmers (especially for OSes)
- Assembly language programmers
- Most importantly: it helps you understand how computers compute
</script></section>
<section>
<h2>Memory Hierarchy, part 1</h2>
<table class="transparent"><tr><td class="top">
<img src="images/07-ibcm/memory-hierarchy.png" alt="memory hierarchy">
</td><td>
<ul>
<li>CPU registers<ul>
<li>1 access per CPU cycle</li>
<li>3*10<sup>9</sup> accesses per sec</li>
<li>1 Kb total storage</li></ul></li>
<li>Cache<ul>
<li>SDRAM: 10 ns</li>
<li>10<sup>8</sup> accesses per sec</li>
<li>Multiple levels possible</li>
<li>Higher levels are bigger and slower</li>
<li>1 Mb total storage</li></ul></li>
</ul>
</td>
</tr></table>
</section>
<section>
<h2>Memory Hierarchy, part 2</h2>
<table class="transparent"><tr><td class="top">
<img src="images/07-ibcm/memory-hierarchy.png" alt="memory hierarchy">
</td><td class="top">
<ul>
<li>Main memory<ul>
<li>DRAM: 60 ns</li>
<li>2*10<sup>7</sup> accesses per sec</li>
<li>Limited by bus speeds</li>
<li>1 Gb total storage</li></ul></li>
<li>Disk<ul>
<li>HDD speeds: 5 ms</li>
<li>200 accesses per sec</li>
<li>1 Tb total storage</li></ul></li>
</ul>
</td></tr></table>
</section>
<section data-markdown><script type="text/template">
## Fetch Execute Cycle
```
while(power is on) {
IR := memory[PC]
Increment PC by length of instruction
execute instruction in IR
}
```
- PC = program counter
- IR = instruction register
</script></section>
<section data-markdown><script type="text/template">
## Assembly Language Instructions
- x86
```
add eax, ebx
add ecx, 1
```
- Explicit use of registers<br> <br>
- IBCM
```
load 100
add 200
store 300
```
- Implied use of accumulator
</script></section>
</section>
<section>
<section id="ibcmdesc" data-markdown><script type="text/template">
# IBCM Description
</script></section>
<section data-markdown><script type="text/template">
## Running IBCM programs
- Online IBCM simulator
- http://www.cs.virginia.edu/~cs216/ibcm/
- Mirrors (best to use in this order):
- http://people.virginia.edu/~asb2t/ibcm/
- http://pegasus.cs.virginia.edu/ibcm/
- http://libra.cs.virginia.edu/ibcm/
- This program will hang your browser if it gets stuck in an infinite loop, due to how web browsers (don't) handle Javascript threads and polling of web pages
</script></section>
<section data-markdown><script type="text/template">
## IBCM Machine Description: CPU
- Single accumulator
- 16 bits
- Special purpose registers
- IR: instruction register
- Stores bits which encode instruction
- PC: program counter
- Stores an address of an instruction
</script></section>
<section>
<h2>IBCM Machine Description: Memory</h2>
<table class="transparent">
<tr><td class="middle"><ul>
<li>4096 16-bit words<ul>
<li>Word<ul>
<li>"chunk size" or addressable unit</li></ul></li>
<li>All initialized to zero initially<ul>
<li>Unlike C/C++</li></ul></li></ul></li>
</ul>
</td><td><img src="images/07-ibcm/ibcm-memory.png" alt="ibcm memory diagram"></td></tr></table>
</section>
<section>
<h2>IBCM instruction format</h2>
<table class="transparent" style="border-collapse:collapse">
<tr>
<td> 15</td>
<td>14</td>
<td>13</td>
<td>12</td>
<td>11 10</td>
<td style="width:300px">...</td>
<td style="text-align:right">0</td>
<td></td>
</tr>
<tr>
<td style="border-top:1px solid; border-left:1px solid" class="middle"> 0</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border-top:1px solid; border-right:1px solid" class="middle">0</td>
<td style="border-top:1px solid"> <br> </td>
<td style="border-top:1px solid; border-bottom:1px solid" class="middle">(unused)</td>
<td style="border-top:1px solid; border-right:1px solid; border-bottom:1px solid"> </td>
<td class="middle">halt</td>
</tr>
<tr>
<td style="border-top:1px solid; border-left:1px solid" class="middle"> 0</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border-top:1px solid" class="middle">1</td>
<td style="border:1px solid">I/O<br>op</td>
<td class="middle">(unused)</td>
<td style="border-right:1px solid"> </td>
<td class="middle">I/O</td>
</tr>
<tr>
<td style="border-top:1px solid; border-left:1px solid" class="middle"> 0</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border-top:1px solid" class="middle">1</td>
<td style="border-top:1px solid" class="middle">0</td>
<td style="border:1px solid"> shift<br>op</td>
<td style="border:1px solid" class="middle">(unused)</td>
<td style="border:1px solid" class="middle"> count</td>
<td class="middle">shifts</td>
</tr>
<tr>
<td colspan="4" style="border:1px solid" class="middle"> opcode</td>
<td style="border-bottom:1px solid"> </td>
<td style="border-bottom:1px solid" class="middle">address</td>
<td style="border-bottom:1px solid; border-right:1px solid"> <br> </td>
<td class="middle"> others</td>
</tr>
</table>
</section>
<section data-markdown><script type="text/template">
## IBCM instruction types
- Halt
- Opcode is 0
- I/O
- Opcode is 1
- next 2 bits specify I/O type
- Shifts
- Opcode is 2
- next 2 bits specify shift type
- last 5 bits specify shift amount
- Others
- Opcode is 3 to F (15)
- Last 12 bits specify the address for the instruction
- The arithmetic, memory, and control instructions
</script></section>
<section data-markdown><script type="text/template">
## Halt
- The opcode (bits 15 down to 12) is zero
- It doesn't matter what the next 12 bits are<br>
- Not much else to say here...<br>
- It halts the IBCM!
</script></section>
<section data-markdown><script type="text/template">
## Input and output
- The opcode (bits 15 down to 12) is 1
- Next two bits specify I/O type:
- Bit 11 specifies input (0) or output (1)
- Bit 10 specifies hex word (0) or ascii (1)
- Combinations:
- 00: read hex word from keyboard
- 01: read ascii character (into acc bits 7-0)
- 10: write hex word to screen
- 11: write ascii character (from acc bits 7-0)
</script></section>
<section data-markdown><script type="text/template">
## Shifts
- The opcode (bits 15 down to 12) is 2
- Next two bits specify shift type:
- Bit 11 specifies shift (0) or rotate (1)
- Bit 10 specifies direction: left (0) or right (1)
- Consider the bits <span class="skyblue">0000</span> <span class="yellow">1111</span> <span class="pink">0000</span> <span class="lightgreen">1111</span> and a 3-bit shift/rotation:
- 00: shift left: <span class="skyblue">0</span><span class="yellow">111 1</span><span class="pink">000 0</span><span class="lightgreen">111 1</span><span class="red">000</span>
- 01: shift right: <span class="red">000</span><span class="skyblue">0 000</span><span class="yellow">1 111</span><span class="pink">0 000</span><span class="lightgreen">1</span>
- 10: rotate left: <span class="skyblue">0</span><span class="yellow">111 1</span><span class="pink">000 0</span><span class="lightgreen">111 1</span><span class="skyblue">000</span>
- 11: rotate right: <span class="lightgreen">111</span><span class="skyblue">0 000</span><span class="yellow">1 111</span><span class="pink">0 000</span><span class="lightgreen">1</span>
</script></section>
<section data-markdown><script type="text/template">
## Other instructions
- Opcode (bits 15 down to 12) varies from 3 to F (15)
- Next 12 bits specifies the address
- See next slide for a list...
</script></section>
<section>
<h2>IBCM "other" instructions</h2>
<table style="font-size:80%;line-height:120%;">
<tr><th>Op</th><th>Name</th><th>HLL meaning</th><th>Description</th></tr>
<tr><td>3</td><td>load</td><td><code>a := mem[addr]</code></td><td>load accumulator from memory</td></tr>
<tr><td>4</td><td>store</td><td><code>mem[addr] := a</code></td><td>store accumulator into memory</td></tr>
<tr><td>5</td><td>add</td><td><code>a := a + mem[addr]</code></td><td>add memory to accumulator</td></tr>
<tr><td>6</td><td>sub</td><td><code>a := a - mem[addr]</code></td><td>add memory from accumulator</td></tr>
<tr><td>7</td><td>and</td><td><code>a := a ∧ mem[addr]</code></td><td>logical 'and' memory into accumulator</td></tr>
<tr><td>8</td><td>or</td><td><code>a := a ∨ mem[addr]</code></td><td>logical 'or' memory into accumulator</td></tr>
<tr><td>9</td><td>xor</td><td><code>a := a ⊕ mem[addr]</code></td><td>logical 'xor' memory into accumulator</td></tr>
<tr><td>A</td><td>not</td><td><code>a := ~a</code></td><td>logical complement of accumulator</td></tr>
<tr><td>B</td><td>nop</td><td><code>/* */</code></td><td>do nothing (no operation)</td></tr>
<tr><td>C</td><td>jmp</td><td><code>goto addr</code></td><td>jump to 'addr'</td></tr>
<tr><td>D</td><td>jmpe</td><td><code>if a == 0 goto addr</code></td><td>jump to 'addr' if accumulator equals zero</td></tr>
<tr><td>E</td><td>jmpl</td><td><code>if a < 0 goto addr</code></td><td>jump to 'addr' if accum. is less than zero</td></tr>
<tr><td>F</td><td>brl</td><td><code>a := PC+1; goto addr</code></td><td>jump (branch) to 'addr'; set accum. to<br>the value of the PC just before the jump<br>(i.e., to the address following the `brl`)</td></tr>
</table>
</section>
<section data-markdown><script type="text/template">
## Labels
- We often need to jump around our code
- For this we use labels in assembly language
- These are translated into absolute memory addresses later by assembler
- We don't have an assembler in IBCM, so we will have to translate them ourselves
- Examples:
```
start readH
loop load n
xit load s
```
</script></section>
<section data-markdown><script type="text/template">
## Declaring variables
- You use the `dw` opcode
- It stands for Declare Word, perhaps
- And you need to give it a label, so you can reference it in assembly language program
- Example:
- `n dw 0` declares a spot with label 'n' to have value 0
- `load n` will load the value stored in that spot in memory
</script></section>
<section>
<h2>IBCM Machine</h2>
<table class="transparent">
<tr><td>
<table class="transparent">
<tr><td>Address</td><td>Contents</td><td>Command</td></tr>
<tr><td>000</td><td class="border">0000</td><td>halt</td></tr>
<tr><td>001</td><td class="border">000f</td><td>halt</td></tr>
<tr><td>002</td><td class="border">0005</td><td>halt</td></tr>
<tr><td>003</td><td class="border">3001</td><td>load mem[1]</td></tr>
<tr><td>004</td><td class="border">5002</td><td>add mem[2]</td></tr>
<tr><td>005</td><td class="border" style="border-bottom:medium solid;">0000</td><td>halt</td></tr>
</table>
</td><td style="width:100px"></td><td>
<table class="transparent">
<tr><td> </td><td></td></tr>
<tr><td>PC</td><td class="border" style="width:100px;border-bottom:medium solid;"> </td></tr>
<tr><td></td><td> </td></tr>
<tr><td>IR</td><td class="border" style="border-bottom:medium solid;"></td></tr>
<tr><td></td><td> </td></tr>
<tr><td>Accum</td><td class="border" style="border-bottom:medium solid;"></td></tr>
</table>
</td></tr></table>
<script type="text/javascript">insertCanvas();</script>
</section>
<section>
<h2>Sample Program</h2>
<table class="transparent">
<tr><td>
<table class="transparent">
<tr><td>Address</td><td>Contents</td><td>Command</td></tr>
<tr><td>000</td><td class="border">3000</td><td class="fragment">load mem[0]</td></tr>
<tr><td>001</td><td class="border">5000</td><td class="fragment">add mem[0]</td></tr>
<tr><td>002</td><td class="border">6001</td><td class="fragment">sub mem[1]</td></tr>
<tr><td>003</td><td class="border">8003</td><td class="fragment">or mem[3]</td></tr>
<tr><td>004</td><td class="border">a000</td><td class="fragment">not</td></tr>
<tr><td>005</td><td class="border">4000</td><td class="fragment">store mem[0]</td></tr>
<tr><td>006</td><td class="border" style="border-bottom:medium solid;">f000</td><td class="fragment">brl mem[0]</td></tr>
</table>
</td><td style="width:100px"></td><td>
<table class="transparent">
<tr><td> </td><td></td></tr>
<tr><td> </td><td></td></tr>
<tr><td>PC</td><td class="border" style="width:100px;border-bottom:medium solid;"> </td></tr>
<tr><td></td><td> </td></tr>
<tr><td>IR</td><td class="border" style="border-bottom:medium solid;"></td></tr>
<tr><td></td><td> </td></tr>
<tr><td>Accum</td><td class="border" style="border-bottom:medium solid;"></td></tr>
</table>
</td></tr></table>
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</section>
<section data-markdown><script type="text/template">
## What It Does
- 0x3000 is a *load* of address 0 into the accum (accum is 3000)
- 0x5000 is a *add* of address 0 to the accum (accum is 6000)
- 0x6001 is a *subtract* of address 1 from the accum (accum is 1000)
- 0x8003 is an *or* of the accum with memory address 3 (accum is 9003)
- 0xA000 is a *not* of the accum (accum is 6FFC)
- 0x4000 is a *store* of the accum into memory 0
- 0xF000 is a *branch and link*
- the PC+1 (0x0007) is stored in the accum
- goto address 0
- Address 0 is now 6FFC
- we've modified our binary code!
- that's a subtract of memory address FFC from the accum
- all memory is initialized to zero, so mem[ffc] = 0
- 0x0007 -- 0x0000 = 0x0007
</script></section>
</section>
<section>
<section id="writingibcm" data-markdown><script type="text/template">
# Writing IBCM Code
</script></section>
<section data-markdown><script type="text/template">
## Writing IBCM Code
- Write high level pseudo-code
```
for ( i = 1; i < max; i++ ) ...
```
- Translate into IBCM assembly instructions
```
load one
store i
```
- Test code by hand
- (step through the code)
- Encode into machine code
```
3016
4005
```
- Load machine code into simulator and run!
</script></section>
<section data-markdown><script type="text/template">
## IBCM Code to compute a sum
- Compute the sum of integers 1 through *n*, where *n* is to be read from the keyboard
- The resulting sum is to be printed to the screen
- Halt after printing the sum
- Source code: [summation.ibcm](../ibcm/summation.ibcm)
- Pseudocode:
```
read n;
i = 1; // index in the array
s = 0; // ongoing sum
while (i <= n) {
s += i;
i += 1;
}
print s;
```
</script></section>
<section data-markdown><script type="text/template">
## IBCM summation program
```
mem locn label op addr comments
C00A 000 jmp start skip around the variables
0000 001 i dw 0 int i
0000 002 s dw 0 int s
0000 003 n dw 0 int n
0001 004 one dw 1
0000 005 zero dw 0
0000 006 leave space for changes
0000 007
0000 008
0000 009
1000 00A start readH read n
4003 00B store n
3004 00C load one i = 1
4001 00D store i
3005 00E load zero s = 0
4002 00F store s
3003 010 loop load n if (i > n) goto xit
6001 011 sub i
E01A 012 jmpl xit
3002 013 load s s += i
5001 014 add i
4002 015 store s
3001 016 load i i += 1
5004 017 add one
4001 018 store i
C010 019 jmp loop goto loop
3002 01A xit load s print s
1800 01B printH
0000 01C halt halt
```
We'll look at this code in parts...
</script></section>
<section data-markdown><script type="text/template">
## IBCM summation program: part 1
```
mem locn label op addr comments
C00A 000 jmp start skip around the vars
0000 001 i dw 0 int i
0000 002 s dw 0 int s
0000 003 n dw 0 int n
0001 004 one dw 1
0000 005 zero dw 0
...
1000 00A start readH read n
4003 00B store n
3004 00C load one i = 1
4001 00D store i
3005 00E load zero s = 0
4002 00F store s
```
</script></section>
<section data-markdown><script type="text/template">
## IBCM summation program: part 2
```
mem locn label op addr comments
3003 010 loop load n if (i > n) goto xit
6001 011 sub i
E01A 012 jmpl xit
3002 013 load s s += i
5001 014 add i
4002 015 store s
3001 016 load i i += 1
5004 017 add one
4001 018 store i
C010 019 jmp loop goto loop
3002 01A xit load s print s
1800 01B printH
0000 01C halt halt
```
</script></section>
<section>
<h2>How Would You Code This?</h2>
<pre><code>if (B == 0)
S1;
else
S2;
</code></pre>
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</section>
<section>
<h2>How Would You Code This?</h2>
<pre><code>while(B >= 5)
S;
</code></pre>
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</section>
<section data-markdown><script type="text/template">
## IBCM Code to Sum Elements in an Array
- Compute the sum of the elements of an array and print this sum on the screen (then halt)
- The address of the first element of the array and the size of the array are to be read in from the keyboard
- Pseudocode:
```
read a; // array base address
read n; // array size
i = 0; // index in the array
s = 0; // ongoing sum
while (i < n) {
s += a[i];
i += 1;
}
print s;
```
- Source code: [array-summation.ibcm](../ibcm/array-summation.ibcm)
</script></section>
<section data-markdown><script type="text/template">
## IBCM Code to Sum Elements in an Array
```
mem locn label op addr comments
C00A 000 jmp start skip around the variables
0000 001 i dw 0 int i
0000 002 s dw 0 int s
0000 003 a dw 0 int a[]
0000 004 n dw 0
0000 005 zero dw 0
0001 006 one dw 1
5000 007 adit dw 5000
0000 008 leave space for changes
0000 009
1000 00A start readH read array address
4003 00B store a
1000 00C readH read array size
4004 00D store n
3005 00E load zero i = 0; s = 0;
4001 00F store i
4002 010 store s
3004 011 loop load n if (i >= N) goto xit
6001 012 sub i
E020 013 jmpl xit
D020 014 jmpe xit
3007 015 load adit form the inst to add a[i]
5003 016 add a
5001 017 add i
401A 018 store doit plant inst into the code
3002 019 load s s += a[i]
0000 01A doit dw 0
4002 01B store s
3001 01C load i i += 1
5006 01D add one
4001 01E store i
C011 01F jmp loop goto loop
3002 020 xit load s print s
1800 021 printH
0000 022 halt
```
We'll look at this code in parts...
</script></section>
<section data-markdown><script type="text/template">
## Array summation: initialization and halting
```
mem locn label op addr comments
C00A 000 jmp start skip around the vars
0000 001 i dw 0 int i
0000 002 s dw 0 int s
0000 003 a dw 0 int a[]
0000 004 n dw 0
0000 005 zero dw 0
0001 006 one dw 1
5000 007 adit dw 5000
... leave space for changes
1000 00A start readH read array address
4003 00B store a
1000 00C readH read array size
4004 00D store n
3005 00E load zero i = 0; s = 0;
4001 00F store i
4002 010 store s
...
3002 020 xit load s print s
1800 021 printH
0000 022 halt
```
</script></section>
<section data-markdown><script type="text/template">
## Array summation: the inner loop
```
mem locn label op addr comments
...
3004 011 loop load n if (i >= N) goto xit
6001 012 sub i
E020 013 jmpl xit
D020 014 jmpe xit
3007 015 load adit form inst to add a[i]
5003 016 add a
5001 017 add i
401A 018 store doit plant inst into code
3002 019 load s s += a[i]
0000 01A doit dw 0
4002 01B store s
3001 01C load i i += 1
5006 01D add one
4001 01E store i
C011 01F jmp loop goto loop
...
```
</script></section>
</section>
<section>
<section id="conclusions" data-markdown><script type="text/template">
# Conclusions
</script></section>
<section data-markdown><script type="text/template">
## Notes
- IBCM's memory
- Array of "words" / chunks of data
- Data can be the program
- Program can be the data
- IBCM is Turing-complete
- Any program expressible in any programming language can be expressed in IBCM
- (with the standard exception about the whole limited-memory thing)
</script></section>
<section data-markdown><script type="text/template">
## IBCM Tips/Reminders
- Use the programming steps
1. Pseudocode
2. Assembly code
- Comment your code clearly!
3. Trace the assembly
4. Translate to machine code... LAST!!!
5. Run in the simulator to verify that it works
- Cannot have blank lines (or comment lines)
</script></section>
<section data-markdown><script type="text/template">
## Use the simulators to debug your code
- Online simulator
- Cannot terminate a infinite loop easily
- Unusual behavior?
- Is the program logic correct?
- Condition specified incorrectly/inaccurately?
- Is the machine code correct?
- Opcode? Address?
</script></section>
<section>
<h2>What's missing from IBCM?</h2>
<ul class="fragment">
<li>Integer multiply and divide</li>
<li>Floating point support</li>
<li>A bigger address space</li>
<li>More than 1 user register</li>
<li>What else?</li>
</ul>
</section>
<section data-markdown><script type="text/template">
## Emulating IBCM in C++
- How might one write software to emulate an IBCM machine in C++?
- A switch statement with 16 cases, perhaps
- But how to decode the instructions?
</script></section>
<section data-markdown><script type="text/template">
## How to decode the parts of an instruction
- Let's assume we had to write a C++ program that could extract the parts of an IBCM instruction
- How to do it?
- Assume the instruction is in an `unsigned int x`
```
unsigned int opcode = (x >> 12) & 0x000f
unsigned int ioshiftop = (x >> 10) & 0x0003
unsigned int address = x & 0x0fff
unsigned int shiftcount = x & 0x000f
```
</script></section>
<section data-markdown><script type="text/template">
## What about encoding
- Assume we have (unsigned ints) opcode, ioshiftop and shiftcount
```
unsigned int instruction = (opcode << 12) |
(ioshiftop << 10) | shiftcount
```
- What a pain this all is!
</script></section>
<section data-markdown><script type="text/template">
## A data structure to make it easier
```
union ibcm_instruction {
#ifdef BIG_ENDIAN // the IBCM is big endian
struct { unsigned char high, low; } bytes;
#else
#ifdef LITTLE_ENDIAN
struct { unsigned char low, high; } bytes;
#else
#error Must define BIG_ENDIAN or LITTLE_ENDIAN
#endif // LITTLE_ENDIAN
#endif // BIG_ENDIAN
struct { unsigned int op:4, unused:12; } halt;
struct { unsigned int op:4, ioopt:2, unused:10 } io;
struct { unsigned int op:4, shiftop: 2,
unused:5, shiftcout:5; } shifts;
struct { unsigned int op:4, address:12; } others;
};
```
</script></section>
<section data-markdown><script type="text/template">
## Using that data structure
```
// read in instruction into unsigned chars a and b
ibcm_instruction inst;
inst.high = a;
inst.low = b;
if ( inst.halt.op == 0 ) { // halt
// ...
} else if ( inst.io.op == 1 ) { // io
cout << inst.io.ioopt << endl;
} else if ( inst.shifts.op == 2 ) { // shifts
cout << inst.shifts.shiftop << endl;
cout << inst.shifts.shiftcount << endl;
} else { // all others
cout << inst.others.address << endl;
}
```
</script></section>
</section>
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