Metal ARM x00 - Setup and Hello World (Lab Notes)

The purpose of Project 0 is to get my own, handwritten machine code running on my practically unused EK-TM4C123GXL board.

Goals

• Flash a minimal, handcrafted, working binary to the board
• No HALs or external software will run on the board apart from what I write
  • I get to use GDB for debugging
  • I get to use external software for flashing and whatever else is needed to get stuff onto the board
• Maximize learning

Work Completed

• Added hx utility for converting hex text -> binary files, and vice versa
  • xxd doesn't support comments, but I wanted to annotate whatever machine code I needed to write and didn't bother looking further
• Created minimal vector table and reset handler
• Initialized stack pointer
• Loop and increment a register forever
• Flashed with openocd and telnet (I think this could have just used openocd)
• Single stepped and verified with gdb

The final program is:

; Vector table should be at 0x0 on reset/power on

; vector table ------------------------------------------
; 0x00 - contains address of initial stack pointer: 0x2000.8000
00 80 00 20

; 0x04 - contains address of reset handler: 0x0000.0008 | 1 (thumb mode set)
09 00 00 00
; -------------------------------------------------------


; Reset handler - Continuously increments R0 ------------------------------------------
; 0x08 - MOV R0 0x01: 0b00100000.00000001 -> 0x2001
01 20 

; loop while incrementing R0
; 0x0A - ADD R0 0x01: 0b00110000.00000001 -> 0x3001
01 30

; branch back to the add
; 0x0C - B to 0x0A (-6 offset in bytes, -3 in halfwords): 0b11100111.11111101 -> 0xE7FD
FD E7
; -------------------------------------------------------------------------------------

Commands Used

• Compile hx to binary: hx program.hxs > program.bin
• Start openocd: openocd -f board/ti_ek-tm4c123gxl.cfg
• Flash binary:

> telnet localhost 4444
> flash write_image erase ./program.bin 0x00000000 bin

• Step and inspect with gdb:

> arm-none-eabi-gdb
(gdb) target extended-remote :3333
Remote debugging using :3333
# ...
(gdb) monitor reset halt
[tm4c123gh6pm.cpu] halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x00000008 msp: 0x20008000
(gdb) stepi
halted: PC: 0x0000000a
(gdb) info registers
r0             0x1                 1
r1             0x0                 0
# ...
pc             0xa                 0xa
# ...

(gdb) stepi
halted: PC: 0x0000000c
(gdb) info registers
r0             0x2                 2
r1             0x0                 0
# ...
pc             0xc                 0xc
# ...

• We can see the register increment forever!

Next steps

• Writing raw machine code is a fun exercise once...time for an assembler