Though we realize that this improvement is perhaps somewhat late
- to be implemented, we believe that there exist better solutions than
-
- marking and suggest a simple modification to the IMP-HOST interface
-
- which would avoid it.
-
- 1 The harm.
-
Marking was introduced to suit the sending Host because it permits
- the text of a message to start on a word boundary, however, it does not
-
- suit the receiving Host with a different word length. Moreover,it
-
- introduces in the message useless bits. Let us illustrate this by the
-
- example of our Sigma 7, a 32 bit machine.
-
- 1.1 Inefficiency in Computation
-
Suppose we receive a message from an 18 bit machine (figure 1.1)
- coded in 8 bit ASCII characters which will eventually become standard on
-
- the network. In order to translate this message into our EBCDIC
-
- internal code, for instance.
-
- 0 17 0 31
-
- -------------------------- ------------------------------
-
- | leader | | leader |
-
- -------------------------- ------------------------------
-
- | | 0 0 0 1| | 0 0 0 1 | |
-
- -------------------------- ----------- |
-
- | | | |
-
- | | | |
-
- | | | |
-
- | message | | message |
-
- | | | |
-
- | | | |
-
- | | | |
-
- | | | |
-
- | | | |
-
- | | | |
-
figure 1.1
[Page 1]
-
- we first have to shift the whole message. We must detect the firsl 1
-
- following the leader, and from this determine that we must shift the
-
- message 4 bits to the left. This takes approximately 12 µsec per double
-
- word, which makes 1,5 msec per full regular message. This is not huge,
-
- but still it is about one-third of the time it will take to translate
-
- the message in internal code.
-
- 1.2 Inefficiency in transmission
-
More important is the inefficiency resulting from adding
- unnecessary bits to the message, especially if it turns out that one
-
- character messages are used. Figure 1.2 shows the example of a 1
-
- character text sent by the sigma 7, which results in transmitting 112
-
- bits to carry 8 bits of information, thus leading to an efficiency
-
- factor of 0.07. Supression of marking would
-
-----------------------------------
Sigma 7 | leader |
-----------------------------------
Message |00000000000000000000000000000001 |
-----------------------------------
| text | 000000000000000000000000 |
-----------------------------------
16 bits of padding | 1000000000000000 |
added by sending IMP --------------------
figure 1.2
- increase this efficiency to 0.10. For a 32 bit text (length of some
-
- control commands), it would increase the efficiency form 0.28 to 0.4.
-
- For one packet messages, the efficiency would still be increased by 3%.
-
- 2 A remedy.
-
This is a suggested modification of the Host-Imp users interface
- which has been tentatively sketched on diagrams extracted form BBN 1822
-
- report.
-
[Page 2]
-
- 2.1 Host to Imp
-
The modification consists of adding a counter to 32, enabled
- as the beginning of a message, and incremented at each bit passed to the
-
- IMP; when it reaches 32 it forces a "word complete" signal asking for a
-
- new word in the shift register and resetting the word length counter;
-
- thus the unused bits in the last word of the leader are not transmitted
-
- and the message starts with the next word (see figure 2.1)
-
0 23
------------------------------------------
| leader |
| ----------------------
| | XXXXXXXXXXXXXXXX | <- contents of
|----------------------------------------- sending Host memory
| | (24 bits)
| Message |
| |
Corresponding message in the sending IMP memory
0 15
--------------------------------
| |
| |
| leader |
| |
--------------------------------
| |
| message |
| |
figure 2.1
- 2.2 Imp to Host
-
The modification consists of adding a counter to 32. When 32 bits
- have entered the shift register form the Imp at the beginning of a new
-
- message, the counter allows the register to be shifted up to the point
-
- to be full (which is detected by the word length counter) without
-
- entering any new bit from the Imp.
-
[Page 3]
-
- Thus, the next bit of the message which is the first bit of text will be
-
- entered as the first bit of the next word (see figure 2.2).
-
- Message in receiving IMP memory Contents of receiving Host memory (35
-
- bits)
-
- 0 15 0 35
-
- ------------------------------ --------------------------------------
-
- | | | |
-
- | leader | | leader | 0000 |
-
- ------------------------------ --------------------------------------
-
- | | | |
-
- | message | | message |
-
- | | | |
-
- | | | |
-
figure 2.2
- Though the accumulated cost of useless marking bits sent over the
-
- network plus computation to reshape received texts makes this
-
- modification probably whorkwhile being considered, this decision is not
-
- of our competence and we merely wanted to suggest a better solution then
-
- marking.
-
Pages 5 and 6 contain a wire Diagram of a
"IMP to Host"
"Host's special Interface"
[ This RFC was put into machine readable form for entry ]
- [ into the online RFC archives by Gottfried Janik 2/98 ]
-
[Page 4]
-