QA871             0                    1                    2                     3                     4                     5                     6                     7                     8                     9

                                          This page performs the timer update routine. The original                                   These words are in all machines whether timer is
                                          contents of the C reg are in D. D is multiplied by                                          installed or not, however, without external hardware
A                                         6 and subtracted from addr 52. If there is no carry                                         (C reg) this routine can never be entered.
                                          out, addr 51 must be decremented. If there is no carry
                                          out, addr 50 must be decremented. If there is still no
                                          carry out, the timer was just decremented thru zero and
                                          F0 must be set to one to cause a timer interrupt  A
                                          carry out at any of the above steps ends the timer update
B                                         with no interrupt indicated.

                                          S0=0 means start next I cycle
                                          S0=1 means return to wait state
C                                         S5=0 means cause a timer int if there was no carry out
                                               during decrement of contents of addr 50

                          

D

                                                    10 --- 0016           00 --- 002C           10 --- 0026           10 --- 001E
                                                |             |       |             |       K 0110        |       K 1101        |
                                                A R+R>D       |       D D+R>D       |       A 0+KH>T      |       A T-KL>T      |
E   QA861.QBE-----------------------------------|             |*------|             |*------|             |*------|             |*---
    (10)                                        |             |       |             |       C LZ>S5       |       |             |   |
    Timer update       D=contents of            |          025V       |             |       |             |       |             |   |
                         C reg                  R 0,0       2CR       R 1,0       26R       R 1,0       1ER       R 0,0       28R   |
                       R=2xD                    E2--    00 --EB       E3--    10 --EC       E4--    10 --ED       E5--    00 --EE   |
                       S0=1,=return to         D=4x orig value       D=6x orig value       Set T=60              T=Lo orde addr     |
F                            wait state                              in D                  Set S5=1              of timer 52)       |
                                                                                                                 S3=1               |
                                                                                                                                    |
                                                                                                                                    |
                                                                                                                                    |
G                                                                                                                                   |
                                                                                                                                    |
                                                                                                                                    |
                                                                                                                                    |
                                                                                                                                    |
                                                                                                                                    |
H                                                                                                                                   |
                                                                                                                                 ---+------------------------------------------------------------------
                                                                                                                                 |  |       00 --- 0028           01 --- 001D           10 --- 002E   |
                                                                                                                                 |  ----|             |       |             |       K 0101        |   |
                                                                                                                                 |      A T-0>T       |       A R-D+C>RC    |       A TXLVKL>D    |   |
J                                                                                                                                O------S T>MN     MS S*--O---|             |*------S WRITE       |*--O
                                                                                                                                 |      |             |   |   |             |       C LZ>S5       |   |
                                                                                                                                 |      |             |   |   |             |       |             |   |
                                                                                                                                 |      R 0,S5      1CR   |   R 1,0       2ER       R S0,S3     28R   |
                                                                                                                                 |      J6--    0* --JF   |   J7--    10 --JG       J8--    ** --JH   |
                                                                                                                                 |                        |  Decrement timer       Note 1             |
K                                                                                                                                |                        |                                           |
                                                                                                                                 |                        |                                           |
                                                                                                                                 |          10 --- 002A   |       00 --- 001C                         |
                                          Note 1-This word sets D=0 and S5=1 as long as an                                       |      |             |   |   |             |                         |
                                              addr starting with 5 is in T. When T hi=4                                          |      A T-0>T       |   |   |             |                         |
L                                             addr 50 was just accessed and if there                                             O------S T>MN     MS S---O---S WRITE       |--------------------------
                                              was not carry out from the decrement of                                            |      |             |       |             |   
                                              its contents, a timer interrupt has just                                           |      |             |       R 1>F0        |   
                                              occurred. The addr in T (4F) is then                                               |      R 0,S5      1CR       R S0,1      29R   
                                              accessed but its contents are left unchanged                                       |      L6--    0* --LF       L7--    ** --LG   
                                              and bit F0 is set to one. If a carry out                                           |                           Timer int, set F0=1
M                                             occurred from the decrement of the contents                                        |                           Addr 4F was just accessed
                                              of addr 50, the update ends with no                                                |                           but not disturbed
                                              interrupt indicated.                                                               |          01 --- 0029                                                       ** --- XXXX
                                                                                                                                 |      K 0001,0      |                                                   | Go to       |
                                                                                                                                 |      |             |                                                   | QA001.CBB   |
N                                                                                                                                O------S         K>W R*--------------------------------------------------|             |
                                                                                                                                 |      |             |                                                   |             |
                                                                                                                                 |      |             |                                                   |             |
                                                                                                                                 |      R S1,INTR   00R                                                   |             |
                                                                                                                                 |      N6--    ** --NF                                                   N9--    ** --NJ
                                                                                                                                 |     End of update                                                     Normal I cycle
P                                                                                                                                |                                                                       start
                                                                                                                                 |
                                                                                                                                 |     
                                                                                                                                 |     
                                                                                                                                 |  
Q                                                                                                                                ------------------------------------------------------------------------------------------------------QA941------JHE
                                                                                                                                                                                                                                       (11)
                                                                                                                                                                                                                                       Go to wait

R

S

 Q
 A
 8
 7                                                                                                                                                | 128015        10/05/65 | Mach          2030       | Date  12/15/65          Sheet    1  QA871 |
 1                                                                                                                                                | 128045        11/17/65 | Name                     | Log    3952             Version    025    |
                                                                                                                                                  | 128047        12/09/65 | Mode          Manual     |                                           |
                                                                                                                                                  |                        | P.N.          837058     |    Interval timer update                  |
                                                                                                                                                  |                        | IBM Corp.     SDD        |    (50 cycle)                             |