When I first got my Altair (early 80s), it had no disks or software.

At first I entered little programs by toggling them into memory through the
front panel switches - but this gets very tedious and difficult once you want
to do more than a few bytes!

Eventually I created "Minimal Code Entry Serial Boot" to write much larger
programs and "bootstrap" them into the Altair via a serial port. All I had
to do now was toggle in 18 bytes!

MCESB.COM is a 16-bit DOS program. It runs well under DosBox
 (I recommend the one available on my site)
 use> MCESB -? for command help

; For these command examples - leading ';' indicatest COMMENT

C:\DOSDIR> NSI NSTAR GET DOS        ; Get NorthStar DOS
C:\DOSDIR> NSI NSTAR GET M2A00      ; "" Monitor
C:\DOSDIR> MCESB -P                 ; Launch MCESB connected to Altair
> B 2000 DOS        ; Load N*DOS into operating position
> B 4000 DOS        ; Another "" because it self-modifies
> B 5000 M2A00      ; Load N*Monitor
> G 20A0            ; Launch in-memory N*DOS
; DOS welcome message displays on ALTAIR
;  See North-Star DOS documentation for meaning of these commands:
* IN 1              ; Initialize disk 1
* CR DOS 10         ; Make DOS file (must be first)
* CR M2A00 8        ; Make M2A00 file
* SF DOS 4000       ; Save DOS
* SF M2A00 5000     ; Save M2A00
* TY M2A00 1 2A00   ; Make M2A00 executable at $2A00
; You can now boot this disk

; If you want you can save the 2nd stage bootloader -!before reboot!-
* CR BOOT2 1
* SF BOOT2 3100
* TY BOOT2 1 3100
; Now, any time after boot you can add another file with
* GO BOOT2
C:\DOSDIR> MCESB -c3 -2
> B 4000 filename
> G 20A0
* CR filename sectors
* SF filename 4000
* TY filename type [execaddr]

; If you want to add other files later without BOOT2
C:\DOSDIR> MCESB -P
> B 4000 filename
> G E900            ; Boot N*DOS
* CR filename sectors
* SF filename 4000
* TY filename type [execaddr]
; Eg: N*BASIC
C:\DOSDIR> NSI NSTAR GET BASIC      ; Get BASIC from Nort
C:\DOSDIR> MCESB
> B 4000 BASIC
> G E900
* CR BASIC 51
* SF BASIC 4000
* TY BASIC 1 2A00


----- ALTAIR and MCESB under DosBox -----
I often test with ALTAIR.COM running on a 2nd instance of DosBox,
which has a very simple DOSBOX.INI
    [cpu]
    cycles=fixed 50000
    [serial]
    serial1=directserial realport:COM1
    serial2=directserial realport:COM2
    serial3=nullmodem server:127.0.0.1 transparent:1 txdelay:1
    [autoexec]
    @echo OFF
    mount C .
    C:
This give direct serial access to COM1 and COM2 while COM3 provides a "serial"
connection to my main DosBox setup which has:
    serial3=nullmodem transparent:1 txdelay:1
on this 2nd DosBox, I ented the command:
    altair l=boot1 W=test c2=3 /g /e
in my main DosBox, I run:
     MCESB -c3
