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     19 <a name=A.7>Table of Contents</a>
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     25 
     26 
     27 <H2>A.7 The optional Block word set</H2>
     28 
     29 Early Forth systems ran stand-alone, with no host OS.  Blocks of 1024
     30 bytes were designed as a convenient unit of disk, and most native Forth
     31 systems still use them.  It is relatively easy to write a native disk
     32 driver that maps head/track/sector addresses to block numbers.  Such
     33 disk drivers are extremely fast in comparison with conventional
     34 file-oriented operating systems, and security is high because there is
     35 no reliance on a disk map.
     36 
     37 <P>
     38 
     39 Today many Forth implementations run under host operating systems,
     40 because the compatibility they offer the user outweighs the performance
     41 overhead.  Many people who use such systems prefer using host OS files
     42 only; however, people who use both native and non-native Forths need a
     43 compatible way of accessing disk.  The Block Word set includes the most
     44 common words for accessing program source and data on disk.
     45 
     46 <P>
     47 
     48 In order to guarantee that Standard Programs that need access to mass
     49 storage have a mechanism appropriate for both native and non-native
     50 implementations, ANS Forth requires that the Block word set be available
     51 if any mass storage facilities are provided.  On non-native
     52 implementations, blocks normally reside in host OS files.
     53 
     54 <P>
     55 
     56 <hr>
     57 <a name=A.7.2>
     58 <H3>A.7.2 Additional terms</H3>
     59 </a>
     60 
     61 <DL><DT><B>block</B>
     62 <DD>Many Forth systems use blocks to contain program source.  Conventionally
     63 such blocks are formatted for editing as 16 lines of 64 characters.
     64 Source blocks are often referred to as <B>screens</B>.</DL>
     65 
     66 <P>
     67 
     68 <hr>
     69 <a name=A.7.6>
     70 <H3>A.7.6 Glossary</H3>
     71 </a>
     72 
     73 
     74 <hr>
     75 <a name=A.7.6.2.2190>
     76 A.7.6.2.2190   SCR
     77 </a>
     78 <P>
     79 
     80 SCR is short for screen.
     81 
     82 <P>
     83 
     84 <hr>
     85 <A href=dpans.htm#toc><IMG   src="up.gif" ></A>    Table of Contents 
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