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95 lines
2.7 KiB
Markdown
95 lines
2.7 KiB
Markdown
# shell
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The shell is a text interface giving you access to commands to control your
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machine. It is not built to be user friendly, but to minimize binary space and
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maximize code simplicity.
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We expect the user of this shell to work with a copy of the user guide within
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reach.
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It is its design goal, however, to give you the levers you need to control your
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machine fully.
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## Commands and arguments
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You invoke a command by typing its name, followed by a list of arguments. All
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numerical arguments have to be typed in hexadecimal form, without prefix or
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suffix. Lowercase is fine. Single digit is fine for byte (not word) arguments
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smaller than `0x10`. Example calls:
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mptr 01ff
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peek 4
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load 1f
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call 00 0123
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All numbers printed by the shell are in hexadecimals form.
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Whenever a command is malformed, the shell will print `ERR` with a code. This
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table describes those codes:
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| Code | Description |
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|------|---------------------------|
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| `01` | Unknown command |
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| `02` | Badly formatted arguments |
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| `03` | Out of bounds |
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| `04` | Unsupported command |
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## mptr
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The shell has a global memory pointer (let's call it `memptr`) that is used by
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other commands. This pointer is 2 bytes long and starts at `0x0000`. To move
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it, you use the mptr command with the new pointer position. The command
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prints out the new `memptr` (just to confirm that it has run). Example:
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> mptr 42ff
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42FF
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## peek
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Read memory targeted by `memptr` and prints its contents in hexadecimal form.
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This command takes one byte argument (optional, default to 1), the number of
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bytes we want to read. Example:
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> mptr 0040
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0040
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> peek 2
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ED56
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## load
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Puts the serial console in input mode and waits for a specific number of
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characters to be typed (that number being specified by a byte argument). These
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characters will be literally placed in memory, one after the other, starting at
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`memptr`.
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This command is, for now, of limited use because it's tied to the active
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console, but a method for selecting I/O sources is planned and this command will
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become much more useful.
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Example:
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> load 5
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Hello
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> peek 5
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48656C6C6F
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## call
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Calls the routine at `memptr`, setting the `A` and `HL` registers to the value
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specified by its optional arguments (default to 0).
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Be aware that this results in a call, not a jump, so your routine needs to
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return if you don't want to break your system.
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The following example works in the case where you've made yourself a jump table
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in your glue code a `jp printstr` at `0x0004`:
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> mptr a000
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A000
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> load 6
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Hello\0 (you can send a null char through a terminal with CTRL+@)
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> mptr 0004
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0004
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> call 00 a000
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Hello>
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