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## A.7 The optional Block word set

Early Forth systems ran stand-alone, with no host OS. Blocks of 1024 bytes were designed as a convenient unit of disk, and most native Forth systems still use them. It is relatively easy to write a native disk driver that maps head/track/sector addresses to block numbers. Such disk drivers are extremely fast in comparison with conventional file-oriented operating systems, and security is high because there is no reliance on a disk map.

Today many Forth implementations run under host operating systems, because the compatibility they offer the user outweighs the performance overhead. Many people who use such systems prefer using host OS files only; however, people who use both native and non-native Forths need a compatible way of accessing disk. The Block Word set includes the most common words for accessing program source and data on disk.

In order to guarantee that Standard Programs that need access to mass storage have a mechanism appropriate for both native and non-native implementations, ANS Forth requires that the Block word set be available if any mass storage facilities are provided. On non-native implementations, blocks normally reside in host OS files.

### A.7.2 Additional terms

block
  Many Forth systems use blocks to contain program source. Conventionally such blocks are formatted for editing as 16 lines of 64 characters. Source blocks are often referred to as screens.

### A.7.6 Glossary

A.7.6.2.2190 SCR

SCR is short for screen.

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