Arithmetic$/$Logic Instruction

The instruction format of the machine $($except LOAD, STORE and BRANCH$)$:

Opcode Source Source Destination

Operand $1$ Operand $2$ Operand

The instructions can be categorized into the following types:

$$ Arithmetic Operations

ADD R$1,$ R$2,$ R$3$ ; R$3<-$ R$1+$ R$2$

SUB R$1,$ R$2,$ R$3$ ; R$3<-$ R$1-$ R$2$

$$ Logical Operations

AND R$1,$ R$2,$ R$3$ ; R$3<-$ R$1$ and R$2$

OR R$1,$ R$2,$ R$3$ ; R$3<-$ R$1$ or R$2$

NOT R$1,$ R$3$ ; R$3<-$ not R$1$

$$ Data Movement Instruction

MOV R$1,$ R$3$ ; R$3<-$ R$1$

Note that in the NOT and MOV operation, source operand $2$ field is not used and will be

set as $00000000.$

$2$

Load$/$Store Instruction

Moving data from Memory to registers and vice versa.

LD A$,$ R$3$ ; R$3<-$ A$,$ A is in memory

ST R$1,$ A ; A $<-$ R$1,$ A is in memory

Load instruction:

Opcode $00000000$ Addressing Destination

$($Load$)$ Mode Operand

Store instruction:

Opcode Source Addressing $00000000($Store$)$ Operand Mode

where the addressing mode $($how to find the target address$)$ is specified in byte $2$ of the

instruction. In this machine, only one addressing mode is used, where the target address

is given by the word following the LOAD or STORE instruction $($Absolute Addressing$).$ This

is specified as $11111111$ in that byte.

Control Instruction

Control flow is by using BRANCH instruction. There are two types of branch instruction

$$ conditional and unconditional Branch. Branch Instruction Format:

Opcode Condition Addressing $00000000($Branch$)$ Code $($cc$)$ Mode

Conditional branch is based on the result of previous ALU operation, which is store in

a flag register. In this machine, we only use a ZERO flag, which will be set to $1$ if the

ALU operation results in $0,$ and set to $0$ otherwise. The target address is specified in

the same way as in memory operation. Similarly, the byte of Addressing Mode is set to

$11111111.$

The condition code is specified as

Condiation Code $($cc$)$ Instruction Description

$00000000$ BR Unconditaion Branch, always goto

$00000001$ BZ Branch if Zero flag is set

$00000010$ BNZ Branch if Zero flag is NOT set

Halt Instruction

The HLT instruction is used to stop the program. The other $3$ bytes are all $0.$