!pr2
!lm12
!rm75
Review:  "The Visible Computer:  6502".....Bob Sander-Cederlof

For five years I have talked about it.  "Someone should write a program that illustrates 6502 code being executed, using hi-res animation."

Software Masters never heard me, but they did it anyway!  "The Visible Computer: 6502" is an animated simulation of our favorite microprocessor.  You see inside the chip and watch the registers change, micro-step by micro-step.  You even see the "hidden" registers:  DL (data latch), DB (data buffer), IR (instruction register), and AD (address).  You see HOW the instructions are executed.

I was amazed at the quality of the documentation.  You get 140 pages of easy-to-follow, fun-to-read tutorial and reference text.  The manual assumes only that you have an Apple, and are moderately familiar with Applesoft.  It doesn't try to teach everything there is to know about machine language, but it does deliver the fundamental concepts.

Thirty demonstration programs are included on the disk, which progressively lead you through the instruction set.  You begin with a two-byte register load, and work up to hi-res graphics and tone generation.  All of the example programs are explained in detail in the manual.  Of course, you can also trace your own programs or programs inside the Apple ROMs.

You can also use the simulator as a debugging tool, if your program will fit in the user memory area.  The simulator provides a 1024-byte user memory, plus a simulated page zero and page one.  You can also use $300-$3CF, if you wish.  One unusual tool for debugging purposes is a full 4-function calculator mode, which works in binary, decimal, or hexadecimal.

Here is a list of the commands available at the normal level:

BASE           select binary, decimal, or hexadecimal
BLOAD          load a program to be simulated
BOOT           boot disk in slot 6, drive 1
CALC           turn on 4-function calculator
EDIT           short-cut entry of hex code into memory
ERASE          clear screen (so graphics can be seen)
L              disassemble five lines of code
LC             select memory for displayed in left column
PRINTER        turn on/off printer in slot one
RC             select memory for display in right column
RESTORE        restore normal screen display
STEP           select one of four simulation modes:
               0 -- fastest, no display update until BRK
               1 -- Full display, simulate until BRK
               2 -- Full display, simulate one instruction
                    with no pause between steps
               3 -- Full display, simulate one instruction,
                    pausing before each step
!np
WINDOW         select one of three display options:
               MEM:  window shows 16 memory cells
               OPEN:  window is blank
               CLOSE:  window shows "hidden" 6502 registers
<addr><value>  store value at memory address
<reg><value>   store value in register


A "MASTER" mode can be turned on, which enables more features and commands for experienced users.  In the master mode you can use the REAL zero page, you can modify any location in memory (even the ones that are dangerous!), you can BLOAD and BSAVE on standard DOS 3.3 disks, and run previously checked subroutines at full 6502 speed.

I know that a lot of you are looking for some help in understanding assembly language;  "The Visible Computer" may be just the help you need.  Let your own Apple teach you!  Some of you are teaching 6502 classes; "The Visible Computer" is the most helpful teaching tools I have ever seen.

I was gratified to learn that the author is an old customer!  He used an older version of the S-C Assembler for coding the longer examples, and the assembly language portions of the simulator.  We even got a free plug on page 108!

The normal retail price of "The Visible Computer" is $49.95, our price will be an even $45 to readers of Apple Assembly Line.
