Appendix B: M U
L TI-LEAVI N
G MULTI-LEAVING is a term that describes a computer-to-computer communication technique developed for use by the HASP system and used by
the RSCS component of VM/310. MULTI-LEAVING can be defined as the fully
synchronized, pseudo-simultaneous, bidirectional transmission of a
variable number of data streams between two or more computers using
binary synchronous communications (BSC) facilities. MULTI-LEAVING in VM/370 The following sections outline the of a comnrehensive. MULTI-LEAVING comllunica tions system (;; - - HASP/ASP) :-. While VM/310 support for programmable BSC remote stations is completely
consistent with the MULTI-LEAVING design, it does not use certain of the
features provided in MULTI-LEAVING: • The transmission of record types other than print, punch, input,
console, and control is not supported. • The only general control record type used is the terminal sign-cn
control. • Multiple data streams are not supported. • Only SCB count units of 1 are used. • No support is included for column binary cards. MULTI-LEAVING Philosophy The basic element for multileaved transmission is the character string. One or more character strings are formed from the smallest external element of transmission, the physical These physical records
are input to MULTI-LEAVING and may be any of the classic record types
(card images, printed lines, tape records, etc.]. For efficiency in
transmission, each of these data records is reduced to a series of character strings of two basic types:
1. A variable-length nonidentical series of characters (for example, SYZ123&1!) • ---and---
2. A variable number of identical characters (for example,
ssssssssss).
An eight-bit control field# termed a String Control Byte (SCB), precedes each character string to identify the type and length of the
string. Thus, a string of nonidentical characters (as in 1 above) represented by an SCB followed by the nonduplicate characters. A strins of conse-cUtlve, - du-plica te, nonblank chara-cters(as in 1 -above) -can-- -be
represented by an SCB and a single character (the seB indicates the
duplication count, and the character following indicates the character
to be duplicated). In the case of an all-blank character string, only an SCB is required to indicate both the type and the number of blank
Appendix B: MULTI-LEAVING 349

characters. A data record to be transmitted is segmented into the
optimum number of character strings (to take full advantage of the
identical character compression) by the transmitting program. A special
SCB is used to indicate the grouping of character strings that compose
the original physical record. The receiving program can then
reconstruct the original record for processing.
Control I Characters I Usage I DLE STX BCB FCS FCS
RCB
SRCB
SCB
DATA
SCB
DATA
SCB
RCB
SRCB
SCB
DATA SCB RCB
DLE
ETB
Figure 48 .• BSC Leader (SOH if no transparency feature)
BSC Start-of-Text
Block Control Byte
Function Control Sequence
Function Control Sequence
Record Control Byte for record 1
Sub-Record Control Byte for record 1
String Control Byte for record 1
Character String
String Control Byte for record 1
Character String
Terminating SCB for record 1
RCB for record 2
SRCB for record 2
SCB for record 2
Character String
Terminating SCB for record 2
Transmission Block terminator BSC Leader (SIN if no transparency feature)
BSC Ending Sequence
A Typical MULTI-LEAVING Transmission Block
In order to allow multiple physical records of various types to be
grouped together in a single transmission block (see Figure 48), an
additional eight-bit control field precedes the group of character
strings representing the original physical record. This the
Record Control Byte identifies the general type and function of the physical record (input stream, print stream, data set, etc.). A
particular RCB type has been designated to allow the passage of control
information betwe.en the various systems. Also, to provide for
simultaneous transmission of similar functions (that is, multiple input
streams, etc.), a stream identification code is included in the RCB. A
second eight-bit control field, the Sub-Record Control Byte (SRCB), is
also included immediately following the RCB. This field is used to
supply additional information concerning the record to the program. For example, in the transmission of data to be printed, the
SRCB can be used for carriage control information.
For actual MULTI-LEAVING transmission, a variable number of records
may be combined into a variable block size, as indicated previously
(that is, RCB,SRCB,SCB1,SCB2, ••• ,SCBn, RCB,SRCB,SCB1, ••• The MULTI-LEAVING design provides for two (or more) computers to exchange
transmission blocks, containing multiple data streams as described
above, in an interleaved fashion. To allow optimum use of this
capability, however, a system must have the capability to control the
flow of a particular data stream while continuing normal transmission cf
all others. This requirement becomes obvious if one considers the case
of the simultaneous transmission of two data streams to a system for
immediate transcription to physical I/O devices of different speeds
(such as two print streams). 350 IBM'VM/370 System Programmer's Guide