🐛 fix XIP sub-word accesses

[stnolting]

This PR fixes the sub-word read access via the XIP (execute in-place) module (#317 by @jpf91).

Tests ✔️

My setup maps the XIP to base address 0x20000000.

Direct test (running a simple memory test program):

XIP data access test...
XIP.word[0x20000000] = 0xfc800513
XIP.word[0x20000004] = 0x00052023

XIP.half[0x20000000] = 0x00000513
XIP.half[0x20000002] = 0x0000fc80
XIP.half[0x20000004] = 0x00002023
XIP.half[0x20000006] = 0x00000005

XIP.byte[0x20000000] = 0x00000013
XIP.byte[0x20000001] = 0x00000005
XIP.byte[0x20000002] = 0x00000080
XIP.byte[0x20000003] = 0x000000fc
XIP.byte[0x20000004] = 0x00000023
XIP.byte[0x20000005] = 0x00000020
XIP.byte[0x20000006] = 0x00000005
XIP.byte[0x20000007] = 0x00000000

Test via GDB:

(gdb) x/wx 0x20000000
0x20000000:     0xfc800513
(gdb) x/wx 0x20000004
0x20000004:     0x00052023

(gdb) x/wx 0x20000001
0x20000001:     0x23fc8005

(gdb) x/bx 0x20000000
0x20000000:     0x13
(gdb) x/bx 0x20000001
0x20000001:     0x05
(gdb) x/bx 0x20000002
0x20000002:     0x80
(gdb) x/bx 0x20000003
0x20000003:     0xfc
(gdb) x/bx 0x20000004
0x20000004:     0x23

[stnolting]

@jpf91

I did some quick tests using the rtl fix from this PR and so far the sub-word accesses seem to be working correctly now. Could you test the modifications, too?

[jpf91]

Thank you for addressing this so quickly!

I can confirm this fixes the issue, for both the reduced test case as well as the real application. Here's the hello_world example running with XIP and our custom XIP bootloader (had to shorten the messages a bit to make it fit in 4K ROM):

<< NEORV32 BLDR >>

BLDV: May 19 2022
HWV:  0x01070108
CLK:  0x05f5e100
ISA:  0x40901104 + 0x00000483
SOC:  0x507f402b
IMEM: 0x00000000 bytes @0x00000000
DMEM: 0x00018000 bytes @0x80000000

Boot in 8s. Press key to abort.
CMD: r, u, e


CMD:> u
Erasing flash... 
Awaiting neorv32_exe.bin... OK (0x000011c8 bytes)
CMD:> e
Booting...

                                                                                      ##        ##   ##   ##    
 ##     ##   #########   ########    ########   ##      ##   ########    ########     ##      ################  
####    ##  ##          ##      ##  ##      ##  ##      ##  ##      ##  ##      ##    ##    ####            ####
## ##   ##  ##          ##      ##  ##      ##  ##      ##          ##         ##     ##      ##   ######   ##  
##  ##  ##  #########   ##      ##  #########   ##      ##      #####        ##       ##    ####   ######   ####
##   ## ##  ##          ##      ##  ##    ##     ##    ##           ##     ##         ##      ##   ######   ##  
##    ####  ##          ##      ##  ##     ##     ##  ##    ##      ##   ##           ##    ####            ####
##     ##    #########   ########   ##      ##      ##       ########   ##########    ##      ################  
                                                                                      ##        ##   ##   ##    
Hello world! :)

And the blink_led example of course also works!

So with this and PR 318 it seems that we can now run any program from XIP flash transparently. :-)

[stnolting]

Great to hear and thanks for the detailed feedback! :D