3.13 Buffers as IO Spaces

We have mentioned already that files are not the only entities that can be edited using poke. Remember the dot-command .file that opened a file as an IO space?

(poke) .file foo.o
The current IOS is now `./foo.o'.
(poke) .info ios
  Id	Type	Mode	Bias		Size		Name
* #0	FILE	rw	0x00000000#B	0x000004c8#B	./foo.o

Memory buffers can be created using a similar dot-command, .mem:

(poke) .mem foo
The current IOS is now `*foo*'.
(poke) .info ios
  Id	Type	Mode	Bias		Size		Name
* #1	MEMORY		0x00000000#B	0x00001000#B    *foo*
  #0	FILE	rw	0x00000000#B	0x000004c8#B    ./foo.o

Note how the name of the buffer is built by prepending and appending asterisks. Therefore, the name of the buffer created by the command .mem foo is *foo*. Note also that the new buffer is created with a default size of 0x1000 bytes, or 4096 bytes. The contents of the buffer are zeroed:

(poke) dump
76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Memory buffer IO spaces grow automatically when a value is mapped beyond their current size. This is very useful when populating newly created buffers. However, for security reasons, there is a limit: the IO spaces are only allow to grow 4096 bytes at a time.

When it comes to map values, there is absolutely no difference between an IO space backed by a file and an IO space backed by a memory buffer. Exactly the same rules apply in both cases.