add-displacement
meaning
microsteps
revised S1 ISA
S1 microprogram
example

                Instruction   addd r1, disp(r2)

Meaning

Let ir:disp be the bit 2..0 of the S-format (3 bit displacement)

Meaning 1  (when the frame pointer itself must be dereferenced)

        R[ir:r1] = R[ir:r1] + M[ ir:disp + M[ R[ir:r2] ] ]

Meaning 2  (when the frame pointer is in r2)

        R[ir:r1] = R[ir:r1] + M[ ir:disp + R[ir:r2] ]

We will assume the second meaning (as it is faster)

Microsteps

<add-disp>
T = ADD( ir:disp, R[ir:r2])
MAR = T
MDR = M[MAR] // memory read
T = ADD( R[ir:r1], MDR )
R[ir:r1] = T

Note: this is not very clean, that RA can be either R[ir:r1] or R[ir:r2].  However it will suffice to illustrate the concept.

We will add the control bits to the microprogram, called, MUX field, MUX = { RB, data }. Also the source field of the databus must add   src + { ir:disp }.   To read a register into port 1 of the alu we need two more control signals: r1p1, r2p1 which read R[ir:r1] to p1  and read R[ir:r2] to p1 respectively.  They are the signal of the selR field.

"trap c" in the pseudo instruction used in the simulator.  We use the instruction "trap 1" to print the content of R[0] to the screen, and "trap 0" to stop the simulation.
 

The revised S1 ISA is

load M r         M -> r
load (r1) r2     M[r1]  -> r2
store r M        r -> M
store r1 (r2)    r1 -> M[r2]
jmp cc A         jump condition
jal r A          jump and link  r store return ads
jr r             jump register
mov r1 r2        r1 -> r2
add r1 r2        r1 = r1 + r2
cmp r1 r2        set cc
inc r1
trap c           trap code c
addd r1 r2 disp  r1 = r1 + M[disp + r2]

op code format
L - format op:3,r0:3,ads:10
S - format op:3,xop:4,r1:3,r2:3,disp:3
M,A = ads
r,cc = r0
c = r1

op inst format
0 load L
1 store L
2 jmp L
3 jal L
4,5,6 undef L
7 xop S

xop inst  format
0 mov S
1 loadr S
2 storer S
3 add S
4 cmp S
5 inc S
6 jr S
7 addd    S
8 trap S
9..15 undef S

condition code bit Z S
0 always
1 EQ  equal   Z = 1
2 NE  not eq  Z = 0
3 LT  S = 1
4 LE  S = 1 or Z = 1
5 GE  S = 0 or Z = 0
6 GT  S = 0

The revised microprogram is:

S1 microprogram

loc label : dest src selR mux alu : Mcnt misc cond goto

29 addd   : - ir:disp r2p1 data add . . .
30        : mar t . . .
31 w5     : - - - - - : rd - mrdy w5
32        : - mdr r1p1 data add . . .
33        : r t ir:r1 - - : - - u ifetch
34 trap   : - - - - - : - - u ifetch

Example of the use of addd instruction

where
r2 holds frame pointer at &STACK
2  is the offset of a local variable no. 2  LV2
LV2 initially 8
r0 initially 1

    .org 0
ld ONE r0          0  0 0 21
ld FP r2           1  0 2 20
addd r0, 2(r2)     2  7 7 0 2 2
trap print         3  7 8 1 0
trap stop          4  7 8 0 0

    .data 20
FP 22              20 22
ONE 1              21 1
STACK 0            22 0
LV1 0              23 0
LV2 8              24 8

The result from executing the above program will be r0 = 9.  This example is in the file "in2.obj".