2.1.2.2 Loading operators

The following instructions are used to load memory contents into a register.

LDA

Put in rA the contents of cell no. M. OPCODE = 8, MOD = fspec. rA <- V.

LDX

Put in rX the contents of cell no. M. OPCODE = 15, MOD = fspec. rX <- V.

LDi

Put in rIi the contents of cell no. M. OPCODE = 8 + i, MOD = fspec. rIi <- V.

LDAN

Put in rA the contents of cell no. M, with opposite sign. OPCODE = 16, MOD = fspec. rA <- -V.

LDXN

Put in rX the contents of cell no. M, with opposite sign. OPCODE = 23, MOD = fspec. rX <- -V.

LDiN

Put in rIi the contents of cell no. M, with opposite sign. OPCODE = 16 + i, MOD = fspec. rIi <- -V.

In all the above load instructions the ‘MOD’ field selects the bytes of the memory cell with address ‘M’ which are loaded into the requisite register (indicated by the ‘OPCODE’). For instance, the word ‘+ 00 13 01 27 11’ represents the instruction

LD3    13,1(3:3)
 ^      ^ ^  ^
 |      | |  |
 |      | |   --- MOD = 27 = 3*8 + 3
 |      |  --- INDEX = 1
 |       --- ADDRESS = 00 13
  --- OPCODE = 11

Let us suppose that, prior to this instruction execution, the state of the MIX computer is the following:

[rI1] = - 00 01
[rI3] = + 24 12
[12] = - 01 02 03 04 05

As, in this case, ‘M = 13 + [rI1] = 12’, we have

V = [M](3:3) = (- 01 02 03 04 05)(3:3)
  = + 00 00 00 00 03

(note that the specified subfield is left-padded with null bytes to complete a word). Hence, the MIX state, after the instruction execution, will be

[rI1] = - 00 01
[rI3] = + 00 03
[12] = - 01 02 03 04 05

To further illustrate loading operators, the following table shows the contents of ‘rX’ after different ‘LDX’ instructions:

‘LDX 12(0:0) [rX] = - 00 00 00 00 00’

‘LDX 12(0:1) [rX] = - 00 00 00 00 01’

‘LDX 12(3:5) [rX] = + 00 00 03 04 05’

‘LDX 12(3:4) [rX] = + 00 00 00 03 04’

‘LDX 12(0:5) [rX] = - 01 02 03 04 05’