Instruction groups, addressing modes, the two-pass assembler, tracing, CPU performance and ports

Last time

Before we start: last time, in Binary shifts and bit manipulation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The language the CPU almost speaks

Machine code is just binary. Assembly language gives each instruction a short name, so a human can read it. An assembler translates it back to binary. Cambridge asks you to read it, trace it, and explain how it is translated. And three nearby topics come with it: what makes a CPU fast, and how devices plug in.

Instruction groups

Data movement: LDM, LDD, LDI, LDX, LDR, MOV and STO load and store data between memory and the accumulator. Arithmetic: ADD, SUB, INC and DEC. Comparison: CMP. Jumps: JMP, JPE and JPN. Input and output: IN and OUT. The program ends with END. Logic and bit manipulation: AND, OR, XOR, and shifts LSL and LSR.

Addressing modes

Immediate: the operand is the data itself. LDM hash 300 puts the number 300 in the accumulator. Direct: the operand is the address of the data. LDD 300 loads what is stored at 300. Indirect: the operand is an address that holds another address. LDI 300 follows the pointer. Indexed: add the index register to the operand to get the address.

The same memory, four loads

Address 300 holds 301, and address 301 holds 47. The index register holds 1. LDM hash 300 gives 300. LDD 300 gives 301, the contents of 300. LDI 300 reads 301 as an address, so it gives 47. LDX 300 uses 300 plus 1, which is 301, so it also gives 47.