[Oberon] FPGA - LONG WORD memory access

[Jörg]

Tomas

If you want to understand how the RISC5 accesses the memory, you can look at
the Verilog files.
- The RISC5.v implements the CPU
- RISC5Top.v implements the CPU's environment (memory, peripherals...)

The CPU (RISC5.v) outputs the variable "adr" to RISC5Top. The task of
RISC5Top.v is to read the memory at address "adr". The following statement
calculates the address for the static RAM:

   assign SRadr = dspreq ? vidadr : adr[19:2];

As you can see, the last two bits adr[1:0] are ignored, so "SRadr" is only a
multiple of 4 even in case the "adr" is odd.

You can verify this with the following program

MODULE Test;
IMPORT S := SYSTEM, Out;
VAR
  p, i: INTEGER;
BEGIN
  p := S.ADR($0001020304050607$);
  S.GET(p, i); Out.Int(i, 10); Out.Ln; INC(p);
  S.GET(p, i); Out.Int(i, 10); Out.Ln; INC(p);
  S.GET(p, i); Out.Int(i, 10); Out.Ln; INC(p);
  S.GET(p, i); Out.Int(i, 10); Out.Ln; INC(p);
  S.GET(p, i); Out.Int(i, 10); Out.Ln; INC(p);
END Test.

Four times the same value is printed (as "p" is handled MOD 4) and only the
last line prints the next value in the ARRAY.

Jörg


-----Original Message-----
From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of Chris
Burrows
Sent: Donnerstag, 27. April 2017 14:16
To: 'ETH Oberon and related systems' <oberon at lists.inf.ethz.ch>
Subject: Re: [Oberon] FPGA - LONG WORD memory access

> -----Original Message-----
> From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of
> Tomas Kral
> Sent: Thursday, 27 April 2017 6:34 PM
> To: Oberon@
> Subject: [Oberon] FPGA - LONG WORD memory access
> 
> Hi,
> 
> Is allowed to address 4byte integers on odd memory address, or does
> it have to be on even boundaries only, aligned on MOD 4?
> 
> I remember from other platforms, namely 68K, words or longs on even
> address only.
> 

Your question is answered in Section 17.1 "The SRAM memory" of the project
Oberon documentation i.e:
 
"The selection of the byte affected is determined by address bits 0 an 1
(which are ignored in the case of word-access)."

Regards,
Chris Burrows
CFB Software
http://www.astrobe.com



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