[Oberon] BYTE to/from INTEGER conversion

[Jörg]

Hi

"Compatible" means you can write
 i := b;
without doing any special kind of operation like  i := LONG(b); or b :=
SHORT(i);

You can mix i and b in the same calculations i := i + b + 5.
You can handover BYTE parameters to procedures expecting INTEGERs.

Jörg


-----Original Message-----
From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of Srinivas
Nayak
Sent: Mittwoch, 6. Juli 2016 18:20
To: ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
Subject: Re: [Oberon] BYTE to/from INTEGER conversion

BYTE is unsigned.
INTEGER is signed.

Oberon7 report says,
"The type BYTE is compatible with the type INTEGER, and vice-versa."
How to interpret the word "compatible" here?


With thanks and best regards,

Yours sincerely,
Srinivas Nayak

Home: http://www.mathmeth.com/sn/
Blog: http://srinivas-nayak.blogspot.in/

On 07/06/2016 08:00 PM, Jörg wrote:
> Hi Srinivas
>
> As you can see in
> https://www.inf.ethz.ch/personal/wirth/Oberon/Oberon07.Report.pdf chapter
> 6.1
> BYTE is unsigned (0..255), INTEGER is signed.
>
> Now, ORB wants to read and store bytes as signed values (-128 and 127)
>
> b has 8 bit, x has 32 bit.
> When writing, the processor just takes the last 8 bit of x.
> If you call ORB.Write(R, -1) it stores the value 0FFH to the file.
> Shortening by cutting bits is automatic.
>
> When reading Files.ReadByte will return b = 255, but ORB wants to see -1.
> The problem with the assignment x := b is that the processor has to know
how
> to fill the upper 24 bit of x. As ORB wants to change from UNSIGNED (b) to
> SIGNED (x) we have to tell the processor, how to fill the upper 24 bits of
> x.
>
> Jörg
>
>
> -----Original Message-----
> From: Oberon [mailto:oberon-bounces at lists.inf.ethz.ch] On Behalf Of
Srinivas
> Nayak
> Sent: Mittwoch, 6. Juli 2016 15:32
> To: ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
> Subject: [Oberon] BYTE to/from INTEGER conversion
>
> In ORB.Mod.txt we see
>
>     PROCEDURE Read(VAR R: Files.Rider; VAR x: INTEGER);
>
>       VAR b: BYTE;
>
>     BEGIN Files.ReadByte(R, b);
>
>       IF b < 80H THEN x := b ELSE x := b - 100H END
>
>     END Read;
>
>
> and
>
>     PROCEDURE Write(VAR R: Files.Rider; x: INTEGER);
>
>     BEGIN Files.WriteByte(R, x)
>
>     END Write;
>
>
>
> Here, Read gets a BYTE from Files.ReadByte(R, b)
> and converts that BYTE to INTEGER using IF .. END
>
> But, Write passes the INTEGER directly to Files.WriteByte(R, x)
> where prototype of WriteByte is PROCEDURE WriteByte*(VAR r: Rider; x:
BYTE);
>
> That means, compiler is able to convert a 4 byte INTEGER implicitly to a 1
> byte BYTE,
> But the reverse is not possible...
>
> How can I understand this?
> Please shed some light on this.
>
>
> With thanks and best regards,
>
> Yours sincerely,
> Srinivas Nayak
>
> Home: http://www.mathmeth.com/sn/
> Blog: http://srinivas-nayak.blogspot.in/
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