It is a long time ago when I used assembly code the last time and I need it now to modify some loader file for a microcontroller.
The file looks like this at the moment:
WD_DISABLE .set 1 ;set to 1 to disable WD, else set to 0
.ref _c_int00
.ref main
.global code_start
.global ExitBoot
***********************************************************************
* Function: codestart section
*
* Description: Branch to code starting point
***********************************************************************
.sect "codestart"
.retain
code_start:
.if WD_DISABLE == 1
LB wd_disable ;Branch to watchdog disable code
.else
LB _c_int00 ;Branch to start of boot._asm in RTS library
;LCR main
.endif
;end codestart section
***********************************************************************
* Function: wd_disable
*
* Description: Disables the watchdog timer
***********************************************************************
.if WD_DISABLE == 1
.text
wd_disable:
SETC OBJMODE ;Set OBJMODE for 28x object code
EALLOW ;Enable EALLOW protected register access
MOVZ DP, #7029h>>6 ;Set data page for WDCR register
MOV @7029h, #0068h ;Set WDDIS bit in WDCR to disable WD
EDIS ;Disable EALLOW protected register access
LB _c_int00 ;Branch to start of boot._asm in RTS library
;LCR main
; Cleanup and exit. At this point the EntryAddr
; is located in the ACC register
BF ExitBoot, UNC
.endif
;end wd_disable
;loadApplication:
; Cleanup and exit. At this point the EntryAddr
; is located in the ACC register
;BF ExitBoot, UNC
;-----------------------------------------------
; ExitBoot
;-----------------------------------------------
;-----------------------------------------------
;This module cleans up after the boot loader
;
; 1) Make sure the stack is deallocated.
; SP = 0x400 after exiting the boot
; loader
; 2) Push 0 onto the stack so RPC will be
; 0 after using LRETR to jump to the
; entry point
; 2) Load RPC with the entry point
; 3) Clear all XARn registers
; 4) Clear ACC, P and XT registers
; 5) LRETR - this will also clear the RPC
; register since 0 was on the stack
;-----------------------------------------------
ExitBoot:
__stack: .usect ".stack",0
;-----------------------------------------------
; Insure that the stack is deallocated
;-----------------------------------------------
MOV SP,#__stack
;-----------------------------------------------
; Clear the bottom of the stack. This will endup
; in RPC when we are finished
;-----------------------------------------------
MOV *SP++,#0
MOV *SP++,#0
;-----------------------------------------------
; Load RPC with the entry point as determined
; by the boot mode. This address will be returned
; in the ACC register.
;-----------------------------------------------
PUSH ACC
POP RPC
;-----------------------------------------------
; Put registers back in their reset state.
;
; Clear all the XARn, ACC, XT, and P and DP
; registers
;
; NOTE: Leave the device in C28x operating mode
; (OBJMODE = 1, AMODE = 0)
;-----------------------------------------------
ZAPA
MOVL XT,ACC
MOVZ AR0,AL
MOVZ AR1,AL
MOVZ AR2,AL
MOVZ AR3,AL
MOVZ AR4,AL
MOVZ AR5,AL
MOVZ AR6,AL
MOVZ AR7,AL
MOVW DP, #0
;------------------------------------------------
; Restore ST0 and ST1. Note OBJMODE is
; the only bit not restored to its reset state.
; OBJMODE is left set for C28x object operating
; mode.
;
; ST0 = 0x0000 ST1 = 0x0A0B
; 15:10 OVC = 0 15:13 ARP = 0
; 9: 7 PM = 0 12 XF = 0
; 6 V = 0 11 M0M1MAP = 1
; 5 N = 0 10 reserved
; 4 Z = 0 9 OBJMODE = 1
; 3 C = 0 8 AMODE = 0
; 2 TC = 0 7 IDLESTAT = 0
; 1 OVM = 0 6 EALLOW = 0
; 0 SXM = 0 5 LOOP = 0
; 4 SPA = 0
; 3 VMAP = 1
; 2 PAGE0 = 0
; 1 DBGM = 1
; 0 INTM = 1
;-----------------------------------------------
MOV *SP++,#0
MOV *SP++,#0x0A0B
POP ST1
POP ST0
;------------------------------------------------
; Jump to the EntryAddr as defined by the
; boot mode selected and continue execution
;-----------------------------------------------
LRETR
;eof ----------
.end
There is the "LCR main" command, that actually loads an address to the stack and gets it back again in the ExitBoot function. Since I somehow have corrupt data when I use the "LCR main" to boot the application, I would like to use "LB _c_int00" and then somehow push the address in the stack, such that the ExitBoot function can load it, but I'm unsure how I can achieve this.
How can I push the address in the ACC register (since I think this is loaded in the ExitBoot) from a C function?
Edit: The architecture is a TMS320C28x 32-bit fixed-point CPU: https://www.ti.com/lit/ug/spru430f/spru430f.pdf?ts=1656867024651&ref_url=https%253A%252F%252Fsoftware-dl.ti.com%252FC2000%252Fdocs%252Foptimization_guide%252Fintro.html
