shell

The shell is a text interface giving you access to commands to control your machine. It is not built to be user friendly, but to minimize binary space and maximize code simplicity.

We expect the user of this shell to work with a copy of the user guide within reach.

It is its design goal, however, to give you the levers you need to control your machine fully.

Commands and arguments

You invoke a command by typing its name, followed by a list of arguments. All numerical arguments have to be typed in hexadecimal form, without prefix or suffix. Lowercase is fine. Single digit is fine for byte (not word) arguments smaller than 0x10. Example calls:

mptr 01ff
peek 4
poke 1f
call 00 0123

All numbers printed by the shell are in hexadecimals form.

Whenever a command is malformed, the shell will print ERR with a code. This table describes those codes:

Code	Description
`01`	Unknown command
`02`	Badly formatted arguments
`03`	Out of bounds
`04`	Unsupported command
`05`	I/O error

Applications have their own error codes as well. If you see an error code that isn't in this list, it's an application-specific error code.

mptr

The shell has a global memory pointer (let's call it memptr) that is used by other commands. This pointer is 2 bytes long and starts at 0x0000. To move it, you use the mptr command with the new pointer position. The command prints out the new memptr (just to confirm that it has run). Example:

> mptr 42ff
42FF

peek

Read memory targeted by memptr and prints its contents in hexadecimal form. This command takes one byte argument (optional, default to 1), the number of bytes we want to read. Example:

> mptr 0040
0040
> peek 2
ED56

poke

Puts the serial console in input mode and waits for a specific number of characters to be typed (that number being specified by a byte argument). These characters will be literally placed in memory, one after the other, starting at memptr.

Example:

> poke 5
Hello
> peek 5
48656C6C6F

call

Calls the routine at memptr, setting the A and HL registers to the value specified by its optional arguments (default to 0).

Be aware that this results in a call, not a jump, so your routine needs to return if you don't want to break your system.

The following example works in the case where you've made yourself a jump table in your glue code a jp printstr at 0x0004:

> mptr a000
A000
> poke 6
Hello\0 (you can send a null char through a terminal with CTRL+@)
> mptr 0004
0004
> call 00 a000
Hello>

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