Add usage guide section about disk blocks

blk/003

@@ -13,4 +13,4 @@ Contents
  4 Number literals              6 Compilation vs meta-comp.
  8 Interpreter I/O             11 Signed-ness
 14 Addressed devices           17 DOES>
-
+18 Disk blocks

blk/018

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+Disk blocks
+
+Disk blocks are Collapse OS' main access to permanent storage.
+The system is exceedingly simple: blocks are contiguous 
+chunks of 1024 bytes each living on some permanent media such
+as floppy disks or SD cards. They are mostly used for text,
+either informational or source code, which is organized into
+16 lines of 64 characters each.
+
+Blocks are referred to by number, 0-indexed. They are read
+through BLK@ and written through BLK!. When a block is read,
+its 1024 bytes content is copied to an in-memory buffer
+starting at BLK( and ending at BLK). Those read/write
+operations are often implicit. For example, LIST calls BLK@.
+
+                                                        (cont.)

blk/019

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+When a word modifies the buffer, it sets the buffer as dirty
+by calling BLK!!. BLK@ checks, before it reads its buffer,
+whether the current buffer is dirty and implicitly calls BLK!
+when it is.
+
+The index of the block currently in memory is kept in BLK>.
+
+Many blocks contain code. That code can be interpreted through
+LOAD. Programs stored in blocks frequently have "loader blocks"
+that take care of loading all blocks relevant to the program.
+