Re: [avr-libc-dev] Error when using large memory addresses

[Paul Schlie]

Unless I misunderstand (and may), there seems to be nothing to work around;
as RAMPZ does not affect RAM load/stores on any devices (I'm aware of), and
is correctly set as required when program memory is accessed; so why would
you want to add code which does nothing useful?


> From: gouy yann <address@hidden>
> Date: Tue, 15 Feb 2005 15:44:34 +0100 (CET)
> To: Per Arnold Blåsmo <address@hidden>, <address@hidden>
> Subject: Re: [avr-libc-dev] Error when using lagre memory addresses
> 
> hello,
> 
> I suspect you will ask to write a bug report for the problem not to be
> forgiven!
> 
> regards.
> 
>    Yann
> 
>  --- Per Arnold Blåsmo <address@hidden> a écrit :
>> Hi,
>> During my work with adapting avg-gcc to a new processor (AT90SC3232CS) I
>> have discovered a problem regarding addressing that I think is not
>> working in the generated code.
>> 
>> I would like some comments on this from some of you more expert than me
>> on this :-)
>> 
>> Consider the following code:
>> ----
>> #include <avr/io.h>
>> #include <avr/pgmspace.h>
>> 
>> #define FROMADDR 0x100000L
>> #define TOADDR1 (uint8_t *)0x200
>> #define TOADDR2    (uint8_t *)0x100
>> 
>> int main(void)
>> {
>>     uint8_t *p_to_addr1= TOADDR1;
>>     uint8_t *p_to_addr2= TOADDR2;
>>     uint32_t from_addr = FROMADDR;
>>     uint8_t size = 255;
>> 
>>     while(size--){
>>         *p_to_addr1++ = pgm_read_byte_far(from_addr++);
>>     }
>>     size=255;
>>     *p_to_addr1 = TOADDR1;
>>     while(size--){
>>         *p_to_addr2++ = *p_to_addr1++;
>>     }
>> 
>> }
>> ----
>> 
>> Compile for an ATmega128 it gives the following assembly:
>> (and yes I know it would not work in real life, but I try to show a
>> point her :-)
>> 
>> ----
>> int main(void)
>> {
>>   62:    cf ef           ldi    r28, 0xFF    ; 255
>>   64:    dc e0           ldi    r29, 0x0C    ; 12
>>   66:    de bf           out    0x3e, r29    ; 62
>>   68:    cd bf           out    0x3d, r28    ; 61
>>     uint8_t *p_to_addr1= TOADDR1;
>>   6a:    00 e0           ldi    r16, 0x00    ; 0
>>   6c:    12 e0           ldi    r17, 0x02    ; 2
>>     uint8_t *p_to_addr2= TOADDR2;
>>   6e:    ee 24           eor    r14, r14
>>   70:    ff 24           eor    r15, r15
>>   72:    68 94           set
>>   74:    f0 f8           bld    r15, 0
>>     uint32_t from_addr = FROMADDR;
>>   76:    40 e0           ldi    r20, 0x00    ; 0
>>   78:    50 e0           ldi    r21, 0x00    ; 0
>>   7a:    60 e1           ldi    r22, 0x10    ; 16
>>   7c:    70 e0           ldi    r23, 0x00    ; 0
>>     uint8_t size = 255;
>> 
>>     while(size--){
>>         *p_to_addr1++ = pgm_read_byte_far(from_addr++);
>>   7e:    ce ef           ldi    r28, 0xFE    ; 254
>>   80:    db 01           movw    r26, r22
>>   82:    ca 01           movw    r24, r20
>>   84:    4f 5f           subi    r20, 0xFF    ; 255
>>   86:    5f 4f           sbci    r21, 0xFF    ; 255
>>   88:    6f 4f           sbci    r22, 0xFF    ; 255
>>   8a:    7f 4f           sbci    r23, 0xFF    ; 255
>>   8c:    98 01           movw    r18, r16
>>   8e:    0f 5f           subi    r16, 0xFF    ; 255
>>   90:    1f 4f           sbci    r17, 0xFF    ; 255
>>   92:    ab bf           out    0x3b, r26    ; 59   <= 0x3b = RAMPZ set
>> to 0x10
>>   94:    fc 01           movw    r30, r24
>>   96:    87 91           elpm    r24, Z+
>>   98:    f9 01           movw    r30, r18
>>   9a:    80 83           st    Z, r24               <= this one should
>> store to 0x0200 but stores to 0x100200 instead
>>   9c:    c1 50           subi    r28, 0x01    ; 1
>>   9e:    80 f7           brcc    .-32         ; 0x80
>>     }
>>     size=255;
>>     *p_to_addr1 = TOADDR1;
>>   a0:    f8 01           movw    r30, r16
>>   a2:    10 82           st    Z, r1
>>     while(size--){
>>         *p_to_addr2++ = *p_to_addr1++;
>>   a4:    ce ef           ldi    r28, 0xFE    ; 254
>>   a6:    f8 01           movw    r30, r16
>>   a8:    81 91           ld    r24, Z+            <= this one should
>> load from 0x0200 but loads from 0x100200
>>   aa:    8f 01           movw    r16, r30
>>   ac:    f7 01           movw    r30, r14
>>   ae:    81 93           st    Z+, r24            <= this one should
>> store in 0x0100 but stores in 0x100100
>>   b0:    7f 01           movw    r14, r30
>>   b2:    c1 50           subi    r28, 0x01    ; 1
>>   b4:    c0 f7           brcc    .-16         ; 0xa6
>>     }
>> 
>> }
>> 
>> -----------
>> 
>> This code will not work as intended.
>> 
>> As you can see I use a 'from address' that is 24 bit put into a long (32
>> bit).
>> To read from this address I use 'pgm_read_byte_far'. 'pgm_read_byte_far'
>> correctly uses
>> RAMPZ (0x3b) to address the memory location ((RAMPZ:Z) = 0x100000).
>> 
>> The problem is that RAMPZ does not get reset after used. That clutters
>> the use of Z later in the code.
>> 
>> My solution to this so far has been to include a line likethis:
>> 
>>     __asm__ volatile ("out     %0, 0x0" : : "I" (_SFR_IO_ADDR(RAMPZ)));
>>     // Hack to reset RAMPZ after it is used
>> 
>> Anyone like to comment on this or/and suggest another work-around?
>> 
>> Per A.
>> 
>> 
>> _______________________________________________
>> AVR-libc-dev mailing list
>> address@hidden
>> http://lists.nongnu.org/mailman/listinfo/avr-libc-dev
>>  
> 
> 
> 
> 
> 
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