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################################################################################

# memtest.s -- Test external RAM memory (XRAM)

#-------------------------------------------------------------------------------

# This program tests the external RAM (connected to the core through the cache

# module). Currently it only finds the RAM top address if given the bottom

# address. Subsequent versions will add some minimal diagnostic capability,

# which will be needed when DRAM access is implemented.

#

# This program does only support a single continuous chunk of RAM. If the cache

# module ever supports more than one chunk (e.g. DRAM and SRAM as in the DE-1

# board) this program will be modified accordingly.

#

# The program assumes there's no useable r/w memory other than the XRAM so it

# does not use any memory for variables or stack.

#

#-------------------------------------------------------------------------------

# To be run from reset vector, standalone. Interrupts must be disabled.

#

################################################################################

    #---- Set to >0 to enable a few debug messages

    .set DEBUG,         0

    #----

    .set XRAM_BASE,     0x80000000          # 1st XRAM address

    .set XRAM_MAX,      1024                # max. no. of KB to test for

    .set UART_BASE,     0x20000000          # UART base address

    .set UART_TX,       0x0000              # TX reg offset

    .set UART_STATUS,   0x0020              # status reg offset

    #---------------------------------------------------------------------------

    .text

    .align  2

    .globl  entry

    .ent    entry

entry:

    .set    noreorder

    b       start_test

    nop

    #--- Trap handler address: we don't expect any traps -----------------------

    .org    0x3c

interrupt_vector:

    b       interrupt_vector

    nop

#-------------------------------------------------------------------------------

start_test:

    mtc0    $0,$12              # disable interrupts

    la      $a0,msg0

    jal     puts

    nop

    li      $a0,XRAM_BASE

    li      $a1,8

    jal     put_hex

    nop

    la      $a0,crlf

    jal     puts

    nop

    la      $t0,XRAM_BASE       # address of memory word being tested

    li      $t2,XRAM_MAX        # max amount of KBs to test for

    li      $t3,1               # (used to decrement $t2)

    li      $t4,0               # no. of KBs found

    move    $t5,$t0             # keep the start addr at hand for comparison

    sw      $zero,0($t0)        # clear 1st test word (in case of prev. run)

test_loop:

    lw      $t1,0($t0)          # read word contents

    beq     $t5,$t1,hit_mirror  # if it's the start address, we hit a mirror

    nop                         # we rolled off the end of the RAM back here

    sw      $t0,0($t0)          # word = word address

    lw      $t1,0($t0)          # read word back...

    bne     $t1,$t0,bad_word    # ...and if no match, we run off the RAM

    nop                         #

    sub     $t2,$t2,$t3         # decrement loop counter...

    bnez    $t2,test_loop       # ...and go back if there's more to go

    addiu   $t0,0x400           # in any case, increment test address by 1KB

    b       end_test            # end of memory found, result is in $t4

    nop

hit_mirror:                     # memory mirror detected

    .ifgt   DEBUG

    la      $a0,msg_mirror

    jal     puts

    nop

    .endif

    b       end_test

    nop

bad_word:                       # readback error detected (maybe r/o area?)

    .ifgt   DEBUG

    la      $a0,msg_bad

    jal     puts

    nop

    .endif

    b       end_test

    nop

end_test:                       # test done, ramtop+1 in $t0, #KB in $t4

    la      $a0,msg1            # Print ramtop message...

    jal     puts

    nop

    move    $a0,$t0

    li      $a1,8

    jal     put_hex

    nop

    la      $a0,crlf

    jal     puts

    nop

$DONE:

    j       $DONE               # ...and freeze here

    nop

#---- Functions ----------------------------------------------------------------

# WARNING: Not for general use!

# All parameters in $a0..$a4, stack unused. No attempt to comply with any ABI

# has been made.

# Since we can't use any RAM, register have been used liberally with no regard

# for intended usage -- have to share reg bank with calling function.

# void puts(char *s) -- print zero-terminated string

puts:

    la      $s0,UART_BASE       # UART base address

puts_loop:

    lb      $v0,0($a0)

    beqz    $v0,puts_end

    addiu   $a0,1

puts_wait_tx_rdy:

    lw      $v1,UART_STATUS($s0)

    andi    $v1,$v1,0x02

    beqz    $v1,puts_wait_tx_rdy

    nop

    sw      $v0,UART_TX($s0)

    b       puts_loop

    nop

puts_end:

    jr      $ra

    nop

# void put_hex(int n, int d) -- print integer as d-digit hex

put_hex:

    la      $s0,UART_BASE

    la      $s1,put_hex_table

    addi    $a1,-1

    add     $a1,$a1,$a1

    add     $a1,$a1,$a1

put_hex_loop:

    srlv    $v0,$a0,$a1

    andi    $v0,$v0,0x0f

    addu    $s2,$s1,$v0

    lb      $v0,0($s2)

put_hex_wait_tx_rdy:

    lw      $v1,UART_STATUS($s0)

    andi    $v1,$v1,0x02

    beqz    $v1,put_hex_wait_tx_rdy

    nop

    sw      $v0,UART_TX($s0)

    bnez    $a1,put_hex_loop

    addi    $a1,-4

    jr      $ra

    nop

#---- Constant data (note we keep it in the text section) ----------------------

put_hex_table:

    .ascii  "0123456789abcdef"

msg0:

    .ascii  "\n\r"

    .asciz  "Scanning external memory at 0x"

msg1:

    .asciz  "Found XRAM top at           0x"

crlf:

    .asciz "\n\r"

space:

    .asciz "  "

msg_mirror:

    .asciz "hit mirror!\n\r"

msg_bad:

    .asciz "bad readback!\n\r"

    .set    reorder

    .end    entry