{"id":573,"date":"2015-11-15T16:13:01","date_gmt":"2015-11-15T16:13:01","guid":{"rendered":"http:\/\/tlindner.macmess.org\/?page_id=573"},"modified":"2015-11-15T16:13:01","modified_gmt":"2015-11-15T16:13:01","slug":"modifying-super-extended-color-basic","status":"publish","type":"page","link":"https:\/\/tlindner.macmess.org\/?page_id=573","title":{"rendered":"Modifying Super Extended Color BASIC"},"content":{"rendered":"

On a recent project I needed to read and write unsigned double byte values from memory. The standard version of BASIC does not include such commands so they must be accomplished using multiple commands. For reading a two byte value this is usually how it is done:\n<pre>\n10 A = PEEK(&H6000)<em>256+PEEK(&H6001)\n<\/pre><\/code>\nVariable A will now contain an unsigned two byte value starting from $6000. This is an example of writing a two byte value:\n<pre>\n10 A=RND(65535)\n20 POKE &H6000,A\/256\n30 POKE &H6001,256<\/em>(A\/256-INT(A\/256))\n<\/pre><\/code>\nIt turns out all this math was causing my recent project to slow down to unacceptable levels. So I had the idea of rewriting the Super Extended Color BASIC commands LPEEK and LPOKE to perform double byte reads and writes. Looking thru the commented source disassembly, I discovered that if it was acceptable to completely replace the functionality of the commands I had 27 bytes for LPEEK and 45 bytes for LPOKE to work with.<\/p>\n

So I took the code from regular PEEK and and placed it where LPEEK was and change the opcode to read two bytes instead of one. This caused a problem, the standard return from a function subroutine converts the D register to a signed value from -32767 to 32766. I wanted an unsigned result from 0 to 65535. In the LIST command I discovered a bit of code that prints the line number to do exactly what I wanted.\n<pre>\n0050 00001 fpa0 equ $50\n002B 00002 binval equ $2b\n00003\n00004 # Modify LPEEK to return two bytes (unsigned)\n00005 org $e573\nE573 17D1CA 00006 lbsr $b740 convert FPA0 to integer in register X\nE576 EC84 00007 ldd ,x read two bytes\nE578 DD50 00008 std fpa0 convert Reg D to FP and return\nE57A C690 00009 ldb #$90\nE57C 43 00010 coma\nE57D BDBC86 00011 jsr $bc86\nE580 39 00012 rts\n<\/pre><\/code>\nThe new LPOKE function needed to read two comma separated values from the program and write the value to the address.\n<pre>\n00014 # Modify LPOKE to store two bytes (unsigned)\n00015 org $e545\nE545 17D1F5 00016 lbsr $b73d eval expression return in X\nE548 9F2B 00017 stx binval store address in binval\nE54A BDB26D 00018 jsr $b26d syntax check for a comma\nE54D 17D1ED 00019 lbsr $b73d eval expression return in X\nE550 1F10 00020 tfr x,d transfer value to D\nE552 9E2B 00021 ldx binval load address\nE554 ED84 00022 std ,x store value in address\nE556 39 00023 rts\n<\/pre><\/code>\nAfter overlaying this code onto SECB, I now have a double byte version of PEEK and POKE. Soon I discovered I had a new slow down. I was filling buffers with the same byte, over and over. Like this:\n<pre>\n10 FOR X = &H6000 TO &H64FF\n20 POKE X, &H55\n30 NEXT X\n<\/pre><\/code>\nThis turned out to be a slow method of filling memory. I needed a new command to quickly accomplish the same thing. I began looking for another command in SECB that I could replace. I decided to replace HPAINT.\n<pre>\n00025 # Modify HPAINT to be a memory fill command\n00026 # Example: HPAINT &H7400, &H2300, &H80\n00027 # Write byte $80 to buffer in $7400 to $96FF\n00028 org $ebf5\nEBF5 17CB45 00029 lbsr $b73d eval expression return in X\nEBF8 9F2B 00030 stx binval store address in binval\nEBFA BDB26D 00031 jsr $b26d syntax check for a comma\nEBFD 17CB3D 00032 lbsr $b73d eval expression return in X\nEC00 3410 00033 pshs x\nEC02 BDB26D 00034 jsr $b26d syntax check for a comma\nEC05 BDB70B 00035 jsr $b70b eval expression return in B\nEC08 9E2B 00036 ldx binval\nEC0A 3520 00037 puls y\n00038 loop\nEC0C E780 00039 stb ,x+\nEC0E 313F 00040 leay -1,y\nEC10 26FA 00041 bne loop\nEC12 39 00042 rts\n00043 end\n<\/pre><\/code>\nTo test it I wrote this screen saver:<\/p>\n

\n10 WIDTH 32:CLS\n20 HPAINT &H3FF+RND(512),RND(128),127+RND(128)\n30 GOTO 20\n<\/code><\/pre>","protected":false},"excerpt":{"rendered":"
On a recent project I needed to read and write unsigned double byte values from memory. The standard version of BASIC does not include such commands so they must be accomplished using multiple commands. For reading a two byte value … Continue reading →<\/span><\/a><\/p>","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-573","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/pages\/573","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=573"}],"version-history":[{"count":13,"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/pages\/573\/revisions"}],"predecessor-version":[{"id":586,"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=\/wp\/v2\/pages\/573\/revisions\/586"}],"wp:attachment":[{"href":"https:\/\/tlindner.macmess.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=573"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}