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//-------------------------------------------------------------------
//
// gp4020_start.S
//
//-------------------------------------------------------------------
//
// Copyright (C) 2003, MLB Associates.        
//
// This file contains basic startup code for the GPS4020 platform.
// After initialization, it will call a single "C" function 'main()'
//
// Note: this program assumes that it has been downloaded into the
// on-chip SRAM via the BOOTSTRAP mechanism, so on-chip SRAM is
// located at address 0x00000000 and external ROM/FLASH is at
// address 0x60000000
//

        .macro  lab,lbl
.\lbl:  .long   \lbl    
        .endm

        .macro  blr
        mov     pc,lr
        .endm
            
        .equ    MPC_BASE_ADDRESS,         0xE0008000
        .equ    MPC_AREA1_CONFIG,         0x00000000
        .equ    MPC_AREA2_CONFIG,         0x00000004
        .equ    MPC_AREA3_CONFIG,         0x00000008
        .equ    MPC_AREA4_CONFIG,         0x0000000C
        .equ    GPIO_BASE_ADDRESS,        0xE0005000
        .equ    GPIO_DIRECTION_REG_OFFSET,0x00000000
        .equ    GPIO_READ_REG_OFFSET,     0x00000004
        .equ    GPIO_WRITE_REG_OFFSET,    0x00000008
        .equ    UART1_BASE_ADDRESS,       0xE0018000
        .equ    UART_TXBUF,               0x00000010
        .equ    UART_STATUS,              0x0000000C
        .equ    UART_STATUS_TXEMPTY,      0x00000002

        .text
        .global __exception_handlers
__exception_handlers:
         b       reset                           // 0x00
         ldr     pc,.undefined_instruction       // 0x04
         ldr     pc,.software_interrupt          // 0x08 start && software int
         ldr     pc,.abort_prefetch              // 0x0C
         ldr     pc,.abort_data                  // 0x10
         .word   0                               // unused
         ldr     pc,.IRQ                         // 0x18
         ldr     pc,.FIQ                         // 0x1C
                                
         .global vectors
vectors:
         .long   reset                           // 0x20
         lab     undefined_instruction           // 0x24
         lab     software_interrupt              // 0x28
         lab     abort_prefetch                  // 0x2C
         lab     abort_data                      // 0x30
         .word   0                               // 0x34
         lab     IRQ                             // 0x38
         lab     FIQ                             // 0x3C

        .arm        @ AKA ".code32"
        .align 2     @ Align code on 2^2 = 4 byte intervals

        .global start
start:
reset:          

/*--------------------------------------------------------------------------*/
// Setup the GPIO and the ARM registers

// make GPIO[0..7] outputs - note: GPIO4 is used for Rx on UART1!
            
        ldr     r1,=GPIO_BASE_ADDRESS
        ldr    r0, [r1,#GPIO_DIRECTION_REG_OFFSET]
        and    r0, r0, #0b00010000    @ 0 = output, 1 = input
        str    r0, [r1,#GPIO_DIRECTION_REG_OFFSET]        @ Store value (DON'T use strb)

        ldr     r1,=MPC_BASE_ADDRESS
        ldr     r2,=0xFF00002D                  // 0x6xxxxxxx, 16bit R/W RAM
        str     r2,[r1,#MPC_AREA1_CONFIG]

        ldr     r1,=MPC_BASE_ADDRESS
        ldr     r2,=0x00000069                  // 0x2xxxxxxx, 16bit R/W RAM
        str     r2,[r1,#MPC_AREA2_CONFIG]

        ldr     r2,[r1,#MPC_AREA3_CONFIG]
        ldr     r2,=0x00000021                  // 0x4xxxxxxx, 16bit peripheral
        str     r2,[r1,#MPC_AREA3_CONFIG]

        ldr     r2,[r1,#MPC_AREA4_CONFIG]
        ldr     r2,=0x0000006E                  // 0x0xxxxxxx, 32bit memory
        str     r2,[r1,#MPC_AREA4_CONFIG]
        
        ldr     sp,=_stack                
        bl      main

10:     ldr     r1,=GPIO_BASE_ADDRESS
        mov     r0,#0
        str    r0, [r1,#GPIO_WRITE_REG_OFFSET]
        b       10b

//
// Exception handlers
//        
undefined_instruction:      // 0x24
        mov     r0,#0x24
        b       exception
software_interrupt:         // 0x28
        mov     r0,#0x28
        b       exception
abort_prefetch:             // 0x2C
        mov     r0,#0x2C
        b       exception
abort_data:                 // 0x30
        mov     r0,#0x30
        b       exception
IRQ:                        // 0x38
        mov     r0,#0x38
        b       exception
FIQ:                        // 0x3C
        mov     r0,#0x3C
        b       exception

exception:
        mov     r8,lr       // Location of failure
        mov     r9,r0
00:             
        ldr     r1,=GPIO_BASE_ADDRESS
        str    r9, [r1,#GPIO_WRITE_REG_OFFSET]
        ldr     r3,=0x100000
10:     nop
        sub     r3,r3,#1
        cmp     r3,#0
        bne     10b
        str     r3,[r1,#GPIO_WRITE_REG_OFFSET]
        ldr     r3,=0x80000
20:     nop
        sub     r3,r3,#1
        cmp     r3,#0
        bne     20b

        ldr     r1,=msg
        bl      _puts
        mov     r1,r8,lsr #24
        bl      _puthex
        mov     r1,r8,lsr #16
        bl      _puthex
        mov     r1,r8,lsr #8
        bl      _puthex
        mov     r1,r8
        bl      _puthex
        mov     r0,#'\n'
        bl      _putc
        mov     r0,#'\r'
        bl      _putc
        b       00b

//
// Write the character in r0
//        
_putc:          
        ldr     r2,=UART1_BASE_ADDRESS
10:     ldrb    r3,[r2,#UART_STATUS]
        and     r3,r3,#UART_STATUS_TXEMPTY
        cmp     r3,#0
        beq     10b
        strb    r0,[r2,#UART_TXBUF]
        blr

//
// Write the string pointed to by r1
//
_puts:
        mov     r7,lr
10:     ldrb    r0,[r1],#1
        cmp     r0,#0
        moveq   pc,r7
        bl      _putc
        b       10b                                        

//
// Print the byte in r1 in hex
//
_puthex:
        mov     r7,lr
        mov     r2,r1,lsr #4
        and     r2,r2,#0x0F
        cmp     r2,#0x0A
        addge   r2,r2,#'A'-'0'-0x0A
        add     r0,r2,#'0'
        bl      _putc
        and     r2,r1,#0x0F
        cmp     r2,#0x0A
        addge   r2,r2,#'A'-'0'-0x0A
        add     r0,r2,#'0'
        bl      _putc
        mov     lr,r7        
        blr

msg:    .string "PC=0x"        
                                            
        .section ".data"
        .align  2
_leds:  .long   0x77        

        .section ".bss"
_stack_base:
        .rept 512
        .byte 0
        .endr
        .balign 16
_stack:

        .end