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|                        01 / Think like an 8051                         |
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An 8051 executes small instructions on bytes and individual bits. The
accumulator A is the main arithmetic register; B assists multiplication
and division. R0-R7 are eight working registers selected from one of four
banks in internal RAM. DPTR is a 16-bit address register, split into DPH
and DPL. PC is the program counter; SP points to the last occupied byte of
the internal-RAM stack.

A byte holds 0-255 unsigned or -128 to +127 in two's-complement notation.
The bit pattern is the same: your program chooses the interpretation.
Hexadecimal 0x2A is decimal 42. In these examples # means an immediate
constant, @ means indirect addressing and a semicolon begins a comment.
DOS/51 accepts 0x-prefixed numbers; many desktop assemblers also accept
2AH. Directives such as ORG, DB, EQU and END are assembler commands, not
CPU instructions, and their spelling depends on the assembler.

The classic CPU has 44 instruction mnemonics, conventionally grouped into
111 operand forms, represented by 255 defined opcode bytes. Opcode A5 is
reserved. Register-number and page-address bits explain why one form can
have several opcodes. This guide covers the original instruction set, not
8051-compatible vendors' later extra opcodes.

One classic machine cycle is 12 oscillator clocks. At the DOS/51 board's
11.0592 MHz that is 1.085 microseconds. Most instructions take one or two
machine cycles; MUL and DIV take four. A modern 1T 8051 derivative is not
timing-compatible. The reference gives classic hardware timing, not a
benchmark of JavaScript execution.

TRY IT IN DOS/51
Open nano first.asm, enter the program below, save with Ctrl-O and leave
with Ctrl-X. Run asm -r first.asm. DOS/51 assembles at 0x8000; RET returns
to the shell. This example prints an asterisk through the board's existing
UART configuration. It does not reinitialize hardware owned by the
operating system.

    clr 0x99         ; clear stale TI before transmitting
    mov a,#42
    mov 0x99,a        ; SBUF: transmit '*'
wait_tx:
    jnb 0x99,wait_tx  ; bit address 0x99 is TI, not SBUF
    clr 0x99         ; acknowledge transmit completion
    ret

That apparent address collision is deliberate: direct byte 0x99 is SBUF,
whereas bit address 0x99 is SCON.1 (TI). The CPU instruction determines
which address space is used.

Bare-metal programs are different: they start at reset address 0x0000,
provide vectors, initialize SP, ports and peripherals, then run forever.
Do not copy a reset-vector program into the DOS/51 transient area and
expect it to own the machine. Save work before experiments that alter the
stack, interrupt state or UART.

[ DOS/51 FIELD MANUAL | IMANIA.DK | 2026-11-14 ]