Differences

This shows you the differences between two versions of the page.

--- its_midas [2022/05/24 01:49]
ldbeth created
+++ its_midas [2022/05/24 02:09]
ldbeth
@@ Line 3: / Line 3: @@
 [[http://pdp10.nocrew.org]] has useful information on the instruction set. I recommend getting familiarized with //DECsystem-10/DECSYSTEM-20 Processor Reference Manual// first.
-The ''MIDAS'' is a macro assembler
+The ''MIDAS'' is a macro assembler, it does not check whether you provide correct number of arguments to opcode.
+Notice that almost all numbers in MIDAS are treated as octal. To specify a decimal number such as nineteen, use ''19.'' (appending a dot).
+You can lookup '':info midas'' for more details about the assembler.
+===== PDP-10 Instruction format =====
+Unlike some CISC instruction set, every instruction of PDP-10 is of the same 36-bit format (except for I/O instructions that are not available when the program is been supervised by an OS).
+<code>
+ 000000000 0111 1 1111 112222222222333333
+ 012345678 9012 3 4567 890123456789012345
+ ________________________________________
+|         |    | |    |                  |
+|   OP    | AC |I| X  |        Y         |
+|_________|____|_|____|__________________|
+        OP  = operation code
+        AC  = accumulator field
+        I   = indirect bit
+        X   = index field
+        Y   = address field
+        DEV = device code
+        IOP = input-output operation code
+</code>
+===== Addressing =====
+Every instruction takes a register argument, and another address argument ''E''.
+Indirect addressing even applies to immediate variant of instructions. For example, the ''movei'' below would actually set register ''a'' to ''12''.
+<code>
+start:  movei  a,@add
+...
+add:    addi   b,12
+</code>
+<code>
+IFETCH: MA <- PC
+        OP <- Bits  0:8  of C(MA);
+        AC <- Bits  9:12 of C(MA);
+EACOMP: I  <- Bit  13    of C(MA);
+        X  <- Bits 14:17 of C(MA);
+        Y  <- Bits 18:35 of C(MA);
+        E  <- Y;
+        IF NOT(X=0) then E <- E+C(X);
+        IF I=0 then go to done;
+        MA <- E;
+        GO TO EACOMP;
+DONE:
+</code>
+===== Print first 500 primes =====
+The ''$'' actually stands for ''^@'' (Control At).
+<code>
+; -*- MIDAS -*-
+        TITLE   PRIME
+L==500. ; Decimal 500
+J=1
+N=2
+K=3
+A=4
+R=5
+PRIME:  BLOCK   L
+BUF:    BLOCK   30
+PDL:    BLOCK   10
+START:  MOVE    J,[-L-1,,PRIME-1]       ; setup J
+        MOVEI   N,2
+        PUSH    J,N
+        MOVEI   N,3     ; N <- 3
+P2:     PUSH    J,N
+        HLRO    A,J
+        AOJE    A,PRIN  ; found
+P4:     ADDI    N,2
+        MOVEI   K,1
+P6:     MOVE    A,N
+        IDIV    A,PRIME(K)
+        JUMPE   R,P4    ; Goto P4 if remainder is zero
+        CAMG    A,PRIME(K)      ; if Q <= P[K]
+        JRST    P2              ; Goto P2
+        ADDI    K,1
+        JRST    P6
+PRIN:   .OPEN   1,[.UAO,,'TTY]
+         .LOSE  %LSSYS
+        MOVE    J,[-10,,PDL-1]
+        MOVE    A,[440700,,TITL]
+        PUSHJ   J,OUT
+        MOVEI   K,0     ; Setup line
+CONV:   MOVE    N,[440700,,BUF]
+NUM:    MOVE    A,PRIME(K)
+        IDIVI   A,1000.
+        PUSHJ   J,DPC
+        IDIVI   A,100.
+        PUSHJ   J,DPC
+        IDIVI   A,10.
+        PUSHJ   J,DPC
+        CAIL    K,450.
+        JRST    PUTL
+        MOVEI   A,40
+        IDPB    A,N
+        ADDI    K,50.
+        JRST    NUM
+PUTL:   MOVE    A,[440700,,BUF]
+        PUSHJ   J,OUT
+        ADDI    K,1
+        CAIL    K,L
+        .LOGOUT 1,
+        SUBI    K,450.
+        JRST    CONV
+DPC:    ADDI    A,60
+        IDPB    A,N
+        MOVE    A,R
+        POPJ    J,
+OUT:    ILDB    N,A
+        JUMPE   N,EXIT
+        .IOT    1,N
+        JRST    OUT
+EXIT:   .IOT    1,[^M]
+        .IOT    1,[^J]
+        POPJ    J,
+TITL:   ASCIZ   $FIRST FIVE HUNDRED PRIMES$
+END     START
+</code>

User Tools

Site Tools

Differences

Page Tools