When additional 4096-word memory banks are attached to the PDP-12, the Memory Extension Control provides access to the additional storage, both for programs and data. The registers of the Control are already built into the PDP-12; they are described in Section 3.22 in relation to LINC mode memory control. In PDP-8 mode, the functions of these registers are the same, but only a portion of each register is used. The Instruction Field (IF), Data Field (DF), and Instruction Field Buffer (IB) registers are each three bits long; the two low-order bits of the 5-bit total pertain only to LINC memory fields. The Save Field register (Interrupt Buffer) is only six bits long; in this case, the four high-order bits are unused.
Instruction Field Register (IF), 3 Bits
These three bits serve as an extension of the PC for determining the 4096-word field from which executable instructions are to be taken. All direct memory references are made to registers in the Instruction Field, with one exception, all JMP and JMS instructions, whether direct or indirect, are registers within the Instruction Field. The exception is the first JMP or JMS executed after a CIF instruction is given. This causes the field to change.
Data Field Register (DF), 3 Bits
These three bits serve as an extension of the Memory Address register for determining which memory bank contains the operands to be accessed by indirect (only) memory references. The Data Field and Instruction Field may be set to the same bank.
Instruction Field Buffer (IB), 3 Bits
This serves as an input buffer for the IF. Except for a direct transfer from the console switches, all transfers into the IF must pass through the IB. When a CIF or RMF instruction is executed, information going to the IF is first placed in the IB. At the next occurrence of a JMP or IMS, the contents of the IB are transferred to the Instruction Field register, and programming continues in the new field, starting in the target register of the jump.
Save Field Register (SF), 6 Bits
Also called the Interrupt Buffer. When a program interrupt occurs, the
contents of the IF and DF are stored in the Save Field register, as
shown in Figure 4-10. After the interrupt has been serviced, an RMF
instruction will cause the contents of the SF to be restored to the DF
and IB. The SF can be examined by using the RIB instruction.
- To Be Done -
Figure 4-10. Data Path to SF and AC
Break Field Register (BF), 3 Bits
When an external device requires extended memory for the transfer of data using the Data Break Facility, the contents of the BF specify the memory bank to be accessed. The use of the register is described in detail in Chapter 5.
All Extended Memory IOT instructions require 4.3 microseconds for execution.
CDF - Change Data Field
Octal code: 62N1, 0 <= N <= 7
Operation: The quantity N is transferred to the Data Field register. All
subsequent indirect memory references by AND, TAD, ISZ, and DCA are to the
new field.
CIF - Change Instruction Field
Octal code: 62N2, 0 <= N <= 7
Operation: The quantity N is transferred to the instruction Field Buffer. At
the occurrence of the next subsequent JMP or JMS instruction, whether direct
or indirect, the contents of the IB are transferred to the IF. The effective
address of the jump is placed in the PC, and the program continues from that
address in the new Instruction Field.
In both CIF and CDF, the number N occupies bits 6-8 of the instruction code.
RDF - Read Data Field
Octal code: 6214
Operation: The contents of the Data Field register are placed in bits 6, 7,
and 8 of the AC. The other bits of the AC are unaffected.
RIF - Read Instruction Field
Octal code: 6224
Operation: The contents of the Data Field register are placed in bits 6, 7,
and 8 of the AC. The other bits of the AC are unaffected.
RIB - Read Interrupt Buffer
Octal code: 6234
Operation: The contents of the Save Field register (Interrupt Buffer) are
transferred to the AC, as follows:
Bits 0-2 (IF) are placed in AC[6:8]; bits 3-5 (DF) are placed in AC[9:11].
RMF - Restore Memory Field
Octal code: 6244
Operation: The contents of the Save Field register are placed in the
instruction Field Buffer and DF as follows: Bits 0-2 (original Instruction
Field) are transferred to the IB; bits 3-5 (original Data Field) are
restored to the Data Field register. This instruction is used to restore
the Memory Field registers after a program interrupt has been serviced.
Normally, the next instruction after the RMF would be JMP I 0; the address
of the interrupted program, stored in register 0000 of bank 0, is placed
in the PC, and the contents of the IB are placed in the Instruction Field
register; the program thus returns to the main program with the Memory Fields
restored to their original values.
All instructions, effective addresses, and directly-addressed operands are taken from the field specified by the contents of the instruction Field Register. All indirectly-addressed operands are taken from (or are stored in) the field specified by the contents of the Data Field Register. The following chart shows the results of the four possible addressing combinations, when the IF and DF designate different memory fields.
Instruction Bits Fields Effecfive Address
Indirect Page IF DF
0 0 m n The operand is in Page 0 of Field m
at the address specified by instruction
bits 5-11.
0 1 m n The operand is in the current page of
Field m.
1 0 m n The effective address of the operand is
in Page 0 of Field m at the
location specified by instruction bits
5-11. The operand is in Field n,
in the location specified by the
effective address.
1 1 m n The effective address is taken from
the current page of Field m, at
the location specified by instruction
bits 5-11. The operand is in Field n.
Autoindexing
When any memory bank is used as an Instruction Field, registers 10-17 of that bank have autoindexing properties, just as the corresponding locations in field 0 do. This is necessary so that a program can operate correctly regardless of the actual memory bank assigned by the IF. When an autoindex register is indirectly addressed, the resulting effective address is used to obtain the operand from the Data Field specified by the DF.
Example:
C(IF)= 2. C(DF)= 4. C(AC)= O.
In field 4: C(4326)= 1107
In field 2: C(0012)= 4325
The instruction, TAD I 12, is executed in field 2.
C(0012)+ 1 -> C(0012). Resulting effective address is 4326.
C(4326) in field 4 are added to the AC
C(AC)= 1107 when the instruction is completed.
Calling A Subroutine Across Fields
The problem is to let the subroutine know which field contains the calling program, so that it can return to the proper point when it's finished. This is most easily done by setting the DF to the same field as the IF, then setting the IF to the field containing the subroutine, and executing a JMS to read the subroutine. The subroutine uses the DF to indirectly obtain data from the calling field, then transfers the C(DF) back to the IF Buffer to return to the calling program. The following example shows a general procedure for doing this.
/CALLING PROGRAM IN FIELD 2, SUBROUTINE IN FIELD 4
/CURRENT DATA FIELD IS 1
/CALLING SEQUENCE SAVES CURRENT DF, PUTS IF IN DF, CALLS
/SUBROUTINE. ON RETURN, ORIGINAL DF IS RESTORED
...
CLA
TAD RESDF /CDF INSTRUCTION TO AC
RDF /C(DF) TO AC 6-8 FORMS CDF 10(6211)
DCA RESDF /STORE IN SEQUENCE TO RESTORE DF
TAD SETDF /CDF TO AC
RIF /C(IF) TO AC 6-8 FORMS CDF 20(6221)
DCA SETDF /STORE IN SEQUENCE TO SET DF
SETDF CDF /SETS DF TO CURRENT IF
CIF 40 /SET IF BUFFER TO SUBROUTINE FIELD 4
JMS I SUBADR /JUMP TO SUBROUTINE IN FIELD 4
RESDF CDF 10 /RESTORES ORIGINAL DF (FIELD 1)
...
SUBADR SUBRTN /ABSOLUTE ADDRESS OF SUBROUTINE
/IN FIELD 4, THE SUBROUTINE HAS THE FOLLOWING GENERAL FORM
SUBRTN 0 /C(PC) FROM CALLING PROGRAM
TAD RESIF /CIF INSTRUCTION TO AC
RDF /C(DF) TO AC6-8 FORMS CIF 20 (6222)
DCA RESIF /STORE IN SEQUENCE TO RESTORE CALLING FIELD
...
...
RESIF CIF 20 /SETS IF BUFFER TO RESTORE CALLING FIELD
JMP I SUBRTN /JUMP BACK TO CALLING PROGRAM
The original contents of the IF, placed in the DF by the calling program, are
used to form a CIF instruction which is placed in the subroutine program
sequence just before the exit, to restore the original calling field.
If, when the interrupt facility is enabled, an interrupt request occurs, the contents of the IF and DF are saved in the Interrupt Buffer. The contents of the PC are stored in register 0000 of Field 0, and the next instruction is taken from register 0001 of Field 0. Regardless of the states of the Memory Field registers, a program interrupt always transfers control to Memory Field 0. When the interrupt has been serviced, the RMF instruction is used to restore the Memory Field registers to their original states. The last three instructions of a general interrupt service routine should be as follows:
RMF /RESTORES DF, PUTS ORIGINAL IF IN IF BUFFER
ION /ENABLE INTERRUPT
JMP I 0 /SETS IF FROM IF BUFFER, AND RETURNS TO
/MAIN PROGRAM.