A side: Controlling the hardware

A cassette tape with a handwritten label: "A Side. Assembly - the first language of the machine."

A A side · Assembly · The first language of the machine

How do you talk to the screen, the keyboard chip and the clock directly?

Flip to the B side (Fortran): Controlling the hardware →

12 Assembly words in this segment

6 Fortran words for the same job

What Assembly says well here

This is the native tongue of the processor. Ports, interrupts and screen memory are all in reach.
It is the best tool to make a retro game fast and responsive.
The helper chips of the PC speak through ports, and the order of the writes is the protocol. To play a note: the command byte B6h to port 43h, then the two bytes of the divisor to port 42h, then bits 0 and 1 of port 61h. A wrong order gives a wrong note or silence.

What is hard to say

Each processor family speaks a different dialect. 8086 words do not work on an ARM processor.
One frame at 70 Hz gives a 4.77 MHz 8086 about 68,000 clock cycles. A fill of the full screen with REP STOSB costs about 640,000. Plan the picture before you draw it.
The VGA has no HDMA, no sprites and no color math. Mode 13h has one page. Mode X has more than one page (four at 200 lines, three at 240 lines), a start address for page flips, and a line compare for a fixed status bar. A per-line effect is work for the 8086 itself, or it does not happen.

One sentence in Assembly

mov dx, 3DAh
vsync: in al, dx
test al, 8
jz vsync

It says: Wait until the screen beam starts its return to the top (vertical retrace).

The words, by part of speech

Verbs (6) · An instruction: one step that the processor does, for example MOV or ADD.

Word	Meaning	Example
IN	Read a value from a hardware port.	`in al, dx`
OUT	Write a value to a hardware port.	`out dx, al`
CLI	Stop hardware interrupts for a short time.	`cli`
STI	Let hardware interrupts start again.	`sti`
IRET	Return from an interrupt handler.	`iret`
HLT	Stop the processor until the next interrupt.	`hlt`

Nouns (6) · A register, a flag, a port or a place in memory, for example AX.

Word	Meaning	Example
port	A numbered door to a hardware device. The processor talks to devices through ports.	`in al, 60h`
interrupt	A signal that stops the program for a moment, runs a handler, then lets the program continue.
A000h	The segment of the VGA screen memory in Mode 13h. Each byte is one pixel.	`mov ax, 0A000h`
3DAh	The VGA status port. Bit 3 is 1 while the screen beam returns to the top (vertical retrace).	`mov dx, 3DAh`
3C8h / 3C9h	The VGA palette ports. Select a color number, then write its red, green and blue parts.	`mov dx, 3C8h`
INT 09h	The keyboard interrupt. A game replaces its handler to read keys fast.

Watch out: what the review found

These notes come from the review of the AI-written guides. They show traps that are easy to miss.

D02-C4 critical (a) Mode 13h runs at 70 Hz. Thus a game locked to vsync gets 70 or 35 fps, not a locked 60. (b) Mode 13h has one page, thus page flipping is not possible. A copy of 64,000 bytes in the ~1 to 1.5 ms vertical blank is not possible over ISA. As a result, tearing becomes less, but it does not stop. Flipping without tears needs Mode X (L11 mentions it, but the roadmap does not use it). (c) The figure in the document (L70), 3.84 MB/s, is more than many 8/16-bit ISA VGA cards can supply.
D02-M8 major 86Box tries to be cycle-accurate. DOSBox-X does not (it uses approximate "cycles"). This is important because performance tuning in DOSBox-X will not match real hardware.
D02-M1 major This is true for 8-bit systems in the 1980s. It is false for the PC in the early 1990s. At that time, C with assembly inner loops was the norm (for example, id Software titles). The source is a Reddit thread [1].
D15-C3 critical The video shows the parallax on the title screen, not in the game [34:08]. The HDMA writes the "background two horizontal scroll register" [35:31], which is $210F (BG2HOFS), not $2111. The table has three bytes per entry: a line count and a two-byte value [35:51]. All band limits, speeds and pixel values on the page are invented. The video names no line numbers. The CPU must build the table and re-enable the channel every frame [36:11], and HDMA itself takes bus cycles from the CPU. The BG3 layer in the game is black tiles [37:13], not mountains. The slider models a screen that does not exist.
D14-M5 major The four addresses $2140 to $2143 are correct. But the video stresses that these are "eight separate registers" (12:28 to 12:48): four per direction, so a write on one side does not erase the value of the other side. The handshake of the boot ROM ($AA and $BB on ports 0 and 1, address on ports 2 and 3, then $CC, 12:48 to 13:48) is the main content of the audio part, and the document omits it. The "16-bit DSP" label at L92 is not in the video.