ARM¶
The ARM platform is a popular CPU architecture. There are 32-bit and 64-bit ARM variants (ARM architecture). Due to the very heterogenous systems found here, only a small subset of 32bit ARM boards may be usable with amForth.
Boot process¶
This describes the bare metal process. The Linux ports run as ordinary programs.
Upon boot, the ARM core reads the first 2 words at address 0 and 4
respectivly. The first number becomes the initial stack pointer
address, the second the initial PC address, effectivly the first
address from which code is executed: The body of the word cold.
cold sets up the Forth VM inner interpreter and hands over to
warm. warm initialzes the remaining system including turnkey
and finally calls quit which never returns and does the usual
Forth command interpreter loop.
CPU – Forth VM Mapping¶
The Forth VM has a few registers that need to be mapped to the controller registers.
Register Mapping¶
| Forth Register | ARM Register |
| W: Working Register | r8 |
| IP: Instruction Pointer | r9 |
| RSP: Return Stack Pointer | r13 (sp) |
| PSP: Parameter Stack Pointer | r7 |
| UP: User Pointer | r10 |
| TOS: Top Of Stack | r6 |
| loopsys (index+limit) | r11 and r12 |
The registers r0 to r5 are currently used as scratch registers. The registers r4 and r5 are planned to be used as extended VM registers.
Memory¶
The memory model is unified. All addresses are available with
the usual @/! words.
Strings are addr/len pairs. Since len is a cell sized number, the length is not really limited. Compiled strings however are limited to be usable with COUNT, that means up to 255 bytes in length.
The memory layout is defined primarily in preamble.inc. It contains
the definitions for the stacks, the first user area and the terminal
input buffer. The dictionary contains further defintions that allocate
RAM. The first unused RAM address can be obtained with here.
Dictionary¶
The dictionary consists of four wordlists. One, forth-wordlist
resides in flash memory and contains all standard words. Another one
called ram-wordlist contains all user defined words. A third
one called arm-wordlist contains ARM specific words. The first
two are in the search order. The ram-wordlist is the current
wordlist too.
Upon reset all words from the ram-wordlist are erased.
> : foo ;
ok
> ram-wordlist show-wordlist
foo ok
> : bar ;
ok
> ram-wordlist show-wordlist
foo bar ok
> cold
amforth 6.8 CORTEX-M4 LM4F120XL
> ram-wordlist show-wordlist
ok
>
Exceptions like -13 for not-found keep this wordlist intact however.