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|                    04 / Practical assembly patterns                    |
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BIT TESTS AND COUNTED LOOPS
    mov r7,#10
again:
    ; body executes ten times
    djnz r7,again
    ret

Starting R7 at zero executes 256 iterations, not zero. DJNZ changes no
arithmetic flags. Conditional branches and SJMP have a signed -128..+127
byte displacement measured from the instruction AFTER the branch. For a
far condition, invert the test and use LJMP. AJMP/ACALL require the target
to share the 2-KB page of the NEXT instruction, so code near a page
boundary needs special care.

UNSIGNED COMPARE
    cjne a,#10,different
    ; A == 10; CY is now 0
    sjmp done
different:
    jc below         ; unsigned A < 10
    ; otherwise A > 10
below:
done:
    ret

PACKED BCD
    mov a,#0x45
    add a,#0x38
    da a             ; A = 83 (hex encoding of decimal 83)
    ret

DA is for the result of packed-BCD ADD/ADDC with valid BCD operands. It is
not binary-to-decimal conversion and is not the correction for
subtraction. DA may set CY; it does not clear an existing carry. AC/OV are
not a decimal result test.

LOOKUP TABLE (DOS/51 PROGRAM)
    mov dptr,#table
    mov a,#2
    movc a,@a+dptr    ; A = ASCII '2'
    ret
table:
    db 48,49,50,51

CODE and XDATA overlap physically only where a BOARD wires them that way.
DOS/51's upper RAM is deliberately visible in both spaces, allowing its
assembler to write instructions with MOVX and then execute them. A generic
80C31 does not create executable RAM by itself.

MULTIPLY / DIVIDE
    mov a,#200
    mov 0xF0,#3      ; B
    mul ab           ; B:A = 0x0258 (600), CY=0, OV=1
    mov a,#200
    mov 0xF0,#7
    div ab           ; A=28 quotient, B=4 remainder, CY=0
    ret

DIV by zero sets OV and leaves A/B results undefined. Check B before
dividing. MUL and DIV are unsigned; signed multiplication/division need
additional sign handling.

ROTATE / SHIFT
RL/RR rotate within A and leave CY alone. RLC/RRC rotate the nine-bit CY:A
chain. For an unsigned right shift use CLR C followed by RRC A. A
multi-byte right shift starts at the high byte; a left shift starts at the
low byte. SWAP A exchanges nibbles; XCH exchanges whole bytes; XCHD
exchanges only the low nibbles of A and an indirectly addressed RAM byte.

MASKS
    anl 0x90,#0xFE   ; clear P1.0, preserve other latches
    orl 0x90,#0x01   ; set P1.0, preserve other latches
    cpl 0x90         ; BIT address 90: toggle P1.0

These examples use numeric SFR/bit addresses for DOS/51 portability. In a
desktop assembler, prefer named EQU/bit declarations from the exact chip
header. Do not assume another assembler's names, macros or directives
exist in the small on-board assembler.

SUBROUTINE CONTRACTS
Document inputs, outputs, scratch registers and flags. Save only what your
calling convention requires, but do not silently change the register bank.
RET restores PC, not A, B, PSW or DPTR. Interrupt entry likewise does not
preserve your working registers. For a callable DOS/51 program, keep the
shell's stack valid and restore any state you borrow. An endless loop,
disabled interrupts or a changed UART can make the shell appear dead;
Reset recovers execution but does not undo files overwritten on the RAM
disk.

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