## AS6100 Assembler

####

    6100 MACHINE DESCRIPTION

       The IM6100 (Intersil) and HM6100 (Harris) microprocessors are
    12-bit word addressable machines having three 12-bit program ac-
    cessible  registers  and one single bit register.  These are the
    Accumulator (AC), MQ Register (MQ), Program  Counter  (PC),  and
    the Link (L) respectively.

       The  6100  is basically a clone of the Digital Equipment Cor-
    poration PDP-8E minicomputer  architecture.   This  architecture
    predates  all  microprocessors  and labeled the bits from 0 (the
    most significant) to 11 (the least significant) rather than from
    least  to  most  significant.  The actual labelling is arbitrary
    and the as6100 assembler uses the now more common labelling.

       The  output  generated from the assembler/linker is two bytes
    per word ordered as MSB then LSB with the upper 4  bits  of  the
    MSB always zero.

####

    ASSEMBLER SPECIFIC DIRECTIVES

       Because  the  6100  microprocessor  has  no  concept of bytes
    several of the cross assembler directives have  their  operation
    changed to reflect the 12-Bit nature of the microprocessor.

    These are:
            .byte           (.db and .fcb are aliases)
                            output an 8-Bit value
                            into a 12-bit word
            .word           (.dw and .fdb are aliases)
                            output a 12-Bit value
                            into a 12-Bit word
            .ascii          (.asciz and ascis also)
                            output a sequence of 8-Bit
                            characters in 12-bit words

       A double precision integer (24-Bits) mnemonic has been added:

            .dubl           (.4byte and .quad are aliases)
                            output a 24-Bit value
                            into two 12-bit words

       Two  new  directives  have  been  added  to  implement  6-bit
    character string operations.  The characters A-Z and  [/]^_  are
    masked  to values of 0x01 to 0x1F, the characters a-z are masked
    to values of 0x01 to 0x1A, and the characters from  ' '  (space)
    to  '?'  are masked to 0x20 to 0x3F.  All other ascii characters
    become a space (0x20).

    These are:
            .text           output upto two characters per 12-bit
    word
            .textz          output upto two characters per 12-bit
    word
                            followed by a 6-bit zero value.

####

    MACHINE SPECIFIC DIRECTIVES

       The  6100  microprocessor  memory architecture consists of 32
    pages each having 128 words for  a  total  of  4096  addressable
    words.   The  6100  instruction set allows direct access only to
    the current page and to page 0.  Three machine  specific  direc-
    tives  provide  differing  methods  to  select  the memory page.
    These directives are:

    .setpg

    Format:

            .setpg          ; . = next page boundary
            .setpg  N       ; . = Nth page boundary

    where:  N       is the page number from 0 to 31

       The .setpg directive is used to set the current program loca-
    tion counter to a specific 128 word page boundary or to the next
    128  word  page boundary and inform the assembler/linker of this
    boundary.

    .mempn

    Format:

            .mempn N        ; . = Nth page boundary

    where:  N       is the page number from 0 to 31

       The .mempn directive is used to set the current program loca-
    tion counter to a specific 128 word page boundary and inform the
    assembler/linker of this boundary.

    .mempa

    Format:

            .mempa A        ; . = A (a page boundary)

    where:  A       is a 128 word page address boundary

       The .mempa directive is used to set the current program loca-
    tion counter to a specific page boundary address and inform  the
    assembler/linker of this boundary.

####

    6100 INSTRUCTION SET

       The  following  tables  list all 6100 family mnemonics recog-
    nized by the AS6100 assembler.  The instruction set is described
    in 3 major groupings:  Basic Instructions, Operate Microinstruc-
    tions, and IOT Instructions.

    Basic Instructions

       The basic instructions are:

            and     Logical AND
            tad     Binary ADD
            isz     Increment and skip if zero
            dca     Deposit and clear AC
            jms     Jump to subroutine
            jmp     Jump

       These instructions have two paging addressing modes:

            addr            current page address
            *addr           page 0 address

    which  can  be  combined with an indirect mode signified by an i
    argument or enclosing brackets []:

            i addr          indirect current page
            [addr]

            i *addr         indirect page 0
            [*addr] or *[addr]

    The  6100  implements  an auto-increment mode when accessing ad-
    dresses 0x08 - 0x0F in page 0 by incrementing  the  contents  of
    the location before using the value as an address.

####

    Operate Instructions

       The  operate  instructions are split into three groups of mu-
    tually exclusive micro operations.  The single  micro  operation
    in common with all three groups is:

            CLA     Clear Accumulator

    Group 1 Operate Instructions  -

       The  group  1 microinstructions are used primarily to perform
    logical operations on the contents of the accumulator and link:

            CLL     Clear Link
            CMA     Complement Accumulator
            CML     Complement Link
            IAC     Increment Accumulator

            RAL     Rotate Accumulator Left
            RTL     Rotate Two Left
            RAR     Rotate Accumulator Right
            RTR     Rotate Two Right
            BSW     Byte Swap

    A group 1 microinstruction can contain one or all of the mnemon-
    ics CLA, CLL, CMA, CML, IAC, but only one of the RAL, RTL,  RAR,
    RTR,  or BSW mnemonics (RAL, RTL, RAR, RTR, and BSW are mutually
    exclusive).

       The  NOP  (No  Operation) functionality can be implemented in
    all three operate instruction groups but is specified by the as-
    sembler as a group 1 instruction.

       Several  common  group 1 operations have been given their own
    mnemonics:

            NOP     NO Operation
            CIA     Complement and Increment Accumulator
            GLT     Get Link
            STA     Set Accumulator

    Group 2 Operate Instructions  -

       The  group 2 microinstructions are used primarily to test the
    contents of the accumulator and/or link and  then  conditionally
    skip the next sequential instruction:

            HLT     Halt
            OSR     Or With Switch Register

            SKP     Skip
            SNL     Skip On Non-Zero Link
            SZL     Skip On Zero Link
            SZA     Skip On Zero Accumulator
            SNA     Skip On Non-Zero Accumulator
            SMA     Skip On Minus Accumulator
            SPA     Skip On Plus Accumulator

    A group 2 microinstruction can contain one or all of the mnemon-
    ics CLA, HLT, OSR, but only one of the SKP, SNL, SZL, SZA,  SNA,
    SMA, or SPA mnemonics (SKP, SNL, SZL, SZA, SNA, SMA, and SPA are
    mutually exclusive).

       One common group 2 operation has been given its own mnemonic:

            LAS     Load Accumulator With Switch Register

    Group 3 Operate Instructions  -

       The  group  3 microinstructions perform logical operations on
    the contents of AC and MQ.

            MQL     MQ Register Load
            MQA     MQ Register Into Accumulator

    A group 3 microinstruction can contain one or all of the mnemon-
    ics CLA, MQL, and MQA.

       Several  common  group 3 operations have been given their own
    mnemonics:

            SWP     Swap Accumulator and MQ Register
            CAM     Clear Accumulator and MQ Register
            ACL     Clear Accumulator and Load
                    MQ Register into Accumulator

    Group Errors  -

       The 6100 assembler has three additional error codes which oc-
    cur when the group 1, 2, or 3 operations are mixed.   The  error
    code  will  be  <1>, <2>, or <3> based upon the first group type
    encountered followed by any other type of group operation.   The
    CLA operation is valid with all groups and does not cause an er-
    ror code to be generated.

####

    Input/Output (IOT) Instructions

       The  input/output  transfer  instructions are used to control
    the operation of peripherals and  transfer  data  between  peri-
    pherals and the 6100 microprocessor.  Of the lower 9 bits of the
    instruction used for device selection and control typically  the
    3  LSBs  are  the  I/O  operation  bits and the remaining 6 bits
    select the peripheral device.

            IOT     DEV,CMND

            where DEV is the device select code and
            CMND is the command code.

    Specifying  a  device select code of zero in the IOT instruction
    allows the user program to control the  interrupt  mechanism  of
    the 6100 microprocessor.  These instructions are:

            SKON    Skip If Interrupt On
            ION     Interrupt Turn On
            IOF     Interrupt Turn Off
            SRQ     Skip If Int Request
            GTF     GetFlags
            RTF     Return Flags
            SGT     Defined By Device Logic
            CAF     Clear All Flags

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###### Last Updated: March 2025